Late gestation pregnancy and placentation

Keywords: bovine, calving, partus, parturition, placenta, caruncles, placentomes, cotyledons, cervix

These images show some of the characteristics of a pregnant uterus removed from a Simmental cow that died suddenly during the early stages of calving. The cause of death was not determined.

As shown below, the calf was located in the right uterine horn. In that regard, it is useful to remind readers that the uterine horn occupied by the fetus is of little significance during cesarean sections. This is because the so called "non-pregnant" horn contains such a small volume of fetal fluid that it is deflected towards the midline by the horn that contains the calf. Therefore (in a normal pregnancy) albeit left or right, the "pregnant" horn is readily accessible from a normal left flank approach.

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In the image above, arrow A indicates that if one were to raise the right uterine horn and inspect the mesometrium (arrow B) the uterine artery would be visible. From about 4 months of gestation, the volume of blood flow in this artery excedes the ability of its thin wall to constrain blood flow in a linear fashion. In bovine pregnancies, this results in turbulent blood flow, reminiscent of air flowing through a "wind sock". In this video, the author illustrates turbulent air flow that is akin to blood flow vibration felt during palpation. This phenomenon is known as fremitus (< Latin. "To murmur"). Fremitus appears later in the vessel supplying the "non-pregnant" horn than that supplying the "pregnant" horn. In both vessels however, it ceases shortly after calving. Therefore it can still be present after calving and in the strictest sense, cannot be regarded as an infallible indicator of pregnancy.

Interestingly, fremitus is not detectable in pregnant mares.

As shown below, the corpus luteum (CL) of pregnancy (ringed in green) was still present in this specimen, shortly before calving would have occurred. This is normal.

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Although the CL of pregnancy is still present at term, most cows remain pregnant even if total ovariectomies are performed within the last 30 days of gestation. Indeed, some cows becomes independent of CL progesterone secretion as early as 6 months of gestation. After ovariectomy however, pregnancies do not end normally. Although pregnancies are maintained to term, cervical dilation, dystocia and retained fetal membranes are common. This is because the corpus luteum is a also a source of relaxin, prostaglandins E1 & E2 and even oxytocin. It is therefore, essential for normal parturition.

Soon (12 to 16 hours) after substantial uterine contractions begin, the cervix relaxes rapidly. This relaxation is largely under the effect of PGE2 but as mentioned earlier, other hormones are also involved.

The initial phase of cervical dilation is largely passive i.e. independent of the dividing force of the fetal head and forelimbs. However, the effect of the fetus can not be excluded completely because the uterus is pushing it towards the cervix, even during the early stages of cervical dilation. Within 8 to 10 hours in cows and somewhat longer in heifers, the cervical canal dilates to a diameter of 12 to 20 centimetres i.e. big enough to accommodate the head of the foetus.

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In the specimen discussed here, passive dilation was incomplete yet almost sufficient to accommodate a human hand (about 12 cm wide, including the thumb).

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Normally, in pre-term pregnant cows, even in autolysed specimens, this would be impossible. Therefore had this cow lived, the second (expulsive) stage of calving would have been imminent.

The image below confirms the fact that the fetus had not yet entered the birth canal i.e. active dilation of the cervix and second stage parturition had not yet begun.

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The following image shows the fetal membranes and the calf, removed from the uterus.  The normal number of placentomes in bovine pregnancies varies between 75 and 120, the largest measuring 12 to 14 cm in length. Also, as reviewed elsewhere, amnionic fluid volume varies between 2 and 8 liters and allantoic fluid, between 4 and 15 liters. This pregnancy appeared to be normal in those respects.

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Placentation in ruminants consists of numerous complexes. Hence the term "multiplex placentation" as apposed to "diffuse, discoidal, zonary" etc.  The two parts of each complex are of course, a caruncle on the maternal side and a cotyledon on the fetal side. Together, they form a placentome. A placentome is shown here:

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As illustrated elsewhere in LORI, the site and number of caruncles in a cow has already been established when the cow herself was a fetus!

Note how the fetal villi are sliding out of the maternal crypts, a process that is impossible if  placental maturation (flattening of maternal epithelium and loosening of fetal-maternal contact) has not occurred during the last few days of gestation. Incomplete placental maturation is a cause of retained placenta. Placental maturation is impaired if the fetus is born prematurely or the diet of the cow is deficient in selenium and vitamin A, nutrients that are essential for epithelial function. This explains in part why retained placenta is common when selenium and vitamin A are deficient in the diet of pregnant cattle.

Incomplete Müllerian duct fusion and canalizing.

Keywords: luteolysis, segmental aplasia, bovine, cervix, mucometra, infertility

A fascinating tract, no doubt from a heifer culled for infertility.

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This tract shows two principal abnormalities. The first is partial aplasia of both uterine horns. However, this aplasia was not in the form commonly encountered, where a segment of one uterine horn is missing. Instead, the Müllerian (paramesonephric) tubes had never canalized to form uterine horns except for a segment in the right Müllerian tube. In that region, a substantial lumen had formed and the uterus contained clear mucoid fluid. The ovaries appeared to be normal and functional, a recent ovulation having occurred from the right ovary. 

The ability of the uterus to produce sufficient amounts of prostaglandin F2 alpha (PGF) to cause luteolysis was of particular interest. In that regard, readers will recall that luteolysis in ruminants depends on production of PGF by the endometrium in the uterine horn ipsilateral to the corpus luteum (CL). In the left ovary of this specimen, there was a small corpus albicans, suggesting that the left uterine horn had indeed produced enough PGF to bring about luteolysis. Yet, there was no apparent lumen in that horn, so the question arises: Were there endometrial mucosal fragments in that horn, not visible to the author? 

It is possible that luteolysis could have occurred by slow (degenerative?) luteolysis in the left ovary. This is known to occur in cattle with complete absence of one uterine horn. In other words, cows or heifers with ovulations ipsilateral to completely aplastic uterine horns do not experience interminable luteal phases.

The second feature of this tract that was particularly interesting was the complete separation of the two uterine horns, including it would appear, their cervixes as well. This is akin to the situation in lagamorphs (rabbits and hares) where the two Müllerian tubes only fuse distal to the cervixes. That is referred to as uterus didelphys (< Gr Delphus meaning womb.). In cattle, it is not rare to encounter two separate or partially fused cervical canals within a large cervical body or even bands of incomplete fusion of the Müllerian system in the vagina but a situation where there are two completely separate Müllerian tubes is indeed rare. Unfortunately the vagina of this specimen was unavailable; it would have been fascinating to see if the cranial vagina (also part of the Müllerian system) was completely separate as well.

An involuting uterus, late postpartum

Keywords: uterus, vagina, bovine, corpus luteum, postpartum, cervix

The uterus of a cow that had involuted partially after calving. She had also ovulated at least once after calving.

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The presence of a corpus luteum (CL) in the left ovary indicates that she had ovulated recently.  Some may ask: Why is it that a cow would ovulate before her uterus is fully involuted and ready to accept a new pregnancy? The answer is that this cow's calf was removed shortly after birth and was not allowed to suckle i.e. this tract was from a dairy cow, a product of human manipulation. Dairy cows are not subjected to the negative effect of sucking on postpartum anestrus. By contrast, beef cows are suckled and do not ovulate for extended periods after calving (depending on breed and nutrition) usually well after the time that the uterus has taken to involute completely.  In the right ovary of this cow, a small follicles has formed a layer of luteal tissue within the follicle, a sign of low level stimulation with luteinizing hormone (LH). Although it is smaller than most cystic follicles it is essentially just that; a luteinized cystic follicle. It is possible that this cystic follicle could have persisted beyond the lifespan of the CL in the left ovary, causing the cow to have a delayed return to estrus (as is often the case). However it is also possible that it was of a similar age to the CL, failing to mature and ovulate as a twin ovulation.  Finally, note that there is no shortage of small tertiary follicles in these ovaries, waiting to participate in postpartum estrous cycles. This is because the production of follicle stimulating hormone (FSH) recovers far sooner that LH production after a calf is born.

The caruncles in this specimen are small and almost completely involuted yet discoloration in the centers on the caruncles suggests that their involution is not complete. The endometrium between caruncles is sloughed soon after calving but is restored by 10 to 12 days postpartum. The caruncles by contrast, have only sloughed tissue to the level of their capillary beds by that time and it is only by about 35 days  postpartum that caruncles are completely restored with new epithelium. Interestingly, caruncles visible throughout the life of the cow. In fact they are even evident when females calves are developing in utero.

Note the short uterine body i.e. how quickly the horns divide as an object ascends into the tract. This is important for several reasons. First, if one is to preferentially inseminate in one horn or another, the pipette must be directed accordingly, as soon as the internal cervical os has been reached, A similar situation is encountered when one has to collect embryos from one horn or another. Finally, because of the virtual absence of a uterine body, bovine fetuses are highly likely to be in longitudinal presentation at calving. By contrast, mares have long uterine bodies making transverse presentation more likely than in cows (although thankfully, transverse presentations are still unusual in mares). Also, no thought is given to horn selection in mares during routine insemination or embryo flushing.

Note the cervical "rings". Apart from the ring that surrounds the external cervical os, these structures are more akin to crescents than complete rings. One must negotiate these rings with an insemination pipettes during artificial insemination (AI). In mares, these rings do not exist and entry into the uterus is simple. In cattle by contrast, the cervix must be grasped per rectum and the tip of the A.I pipette placed in the fornix of the vagina. Then the external cervical os is then manipulated over the instrument while the instrument itself is eased through the cervical canal. The same situation is encountered when one flushes embryos from the uterus.

Interestingly, the word fornix is derived from the Latin word for archway. In homo sapiens, standing erect, this is perhaps appropriate. In domestic animals however, the arches (around the cervix) have fallen on their sides!

Selected reference:

The mind of an aging theriogenologist.

Cystic structures in the vagina

Keywords: vagina, bovine, Gartner's, Bartholin, Nabothian, minor, major, vestibular, mesonephric, duct, cervix, Skene

First, a reminder as to the location of the structures under discussion in this entry. The vagina in this context includes the cranial vagina and the vestibule of the vagina i.e. that segment caudal to the external urethral opening. The author has constructed the following drawing to illustrate the position of various vaginal structures.

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The structures discussed here bear the names of surgeons, gynecologists or anatomists: Thomas Bartholin (Bartholin's gland), Hermann Gärtner (the umlaut is usually omitted i.e Gartner; Gartner's ducts) and Martin Naboth (Nabothian glands). There is also brief mention of  the eponyn Skene's glands, named for Alexander Skene.

The image below shows two small Nabothian cysts in the mucosa of the external cervical os of a cow. A yellow margin has been drawn around them to emphasize their location. Nabothian cysts containing thick, tenacious mucus can be several times larger than those seen here; large enough to be palpable through the vaginal wall.

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Nabothian cysts also occur in water buffalo (Bubalus bubalus), humans and probably many other animals. In water buffalo they occurred at a frequency of approximately one percent in one study but in domestic cattle, the incidence is probably much lower.  In 35 years of examining large numbers of bovine tracts from slaughter plants, the author has seen fewer than a half a dozen Nabothian cysts.

The cervix has a large population of Nabothian mucus-secreting glands. They provide lubrication during coitus and a cervical seal during pregnancy. It is when the secretory ducts of these glands become occluded, that Nabothian cysts develop. The appearance and disappearance of these cysts in relation to the menstrual cycle in women suggests, not surprisingly, that they respond to variations in estrogens during the cycle. The same is probably true of cattle where copious amounts of mucus are produced by the cervical glands during estrus but little, and of a more tenacious nature, during the luteal phase.

Unless Nabothian cysts  are very large and impede copulation, sperm transport or even artificial insemination, their effect on fertility is insignificant.

Below: A drawing of cystic Gartner's ducts constructed by the author from several images and personal observations. In this regard, a high quality image of cystic Gartners ducts would be most welcome in this entry. If the reader has such an image (one that could be lent to this library) please contact the author (lofstedt@upei.ca).

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Albeit a minor point, this author suggests that the term "hydroptic" (< hydrops) may be more appropriate than "cystic" in this case. The former description is akin to the term hydropsalpinx; used for distension of the uterine tube. In the author's opinion, the term "cystic" is more reminiscent of a distended sphere or sphere-like structure than a tubular structure. A trivial matter to be sure.

Gartner's ducts are remnants of the mesonephric (Wolffian) ducts present in bi-potential early embryos. These ducts fail to develop into the deferent ducts in the absence androgens from testicles. Gartners ducts themselves do not appear to have a significant secretory function. However, as described in an investigation of the reproductive tracts of 70 cattle, Gartner's ducts are closely associated with, or joined to, partially developed glands at the caudal openings of Gartner's ducts where they entered the vagina. The occasional presence of minor vestibular glands at the base of Gartners ducts in cattle may explain why the ducts sometimes become distended when their openings into the vagina are occluded. Current knowledge suggests that these glands are analogous to the prostate gland in males. They are referred to as minor vestibular glands or in human literature, as Skene's glands or even female prostate glands.

Note: Alexander Skene's eponym is not used in veterinary literature, probably because the minor vestibular glands (Skene's glands) are poorly developed in most animals. In humans by contrast, Skene's glands are usually well developed and are thought to be important for lubrication during sexual intercourse.

In one study of over a 1000 bovine reproductive tracts cystic Gartner's ducts were present in approximately 1% of the samples in other words, cystic Gartner's ducts are not common. There are rare reports of grossly distended Gartner's ducts. In one case, a duct was distended to the diameter of a grape fruit.

In most cases, cystic Gartner's ducts do not interfere with reproduction

Some readers  may wonder why the openings of Gartners ducts are so seldom seen during per vagina examinations. This is explained by by the fact that the openings of Gartner's ducts are very small, almost invisible or in some cases, absent altogether. In three different studies reviewed by this author, there were substantial differences in the presence or absence (unilaterally or bilaterally) of openings to Gartners ducts in cows. In essence, their openings were present bilaterally in perhaps 50 to 70% of cows and a single left duct-opening was present more often than a single right duct-opening.

In the case of Nabothian cysts, and cystic conditions of Gartners ducts and Bartholin's glands it is often not known what caused the openings of these ducts or glands to become occluded. It is possible of course, that contraction of scar tissue may be responsible for this after local inflammation. However, the openings of these ducts or glands could also become occluded due to hyperkeratosis in cases of hypo-vitaminosis A. Certainly, a lack of vitamin A has been shown to cause vaginal hyperkeratinosis in mice.  Interestingly, reports of hyperkeratosis in the genital tract began to emerge after 1947 when chlorinated naphalene (used in petroleum products and wood preservatives at the time) was found in cattle feed.  Not surprisingly, Dr S.J. Roberts in his masterpiece "Veterinary obstetrics and genital diseases. Theriogenology" refers to a 1953 paper by his Cornell colleague Dr K. McEntee, stating that "....abnormalities of Gartner's ducts...are rather common in the cow".  Clorinated naphlalenes are no longer common in cattle feed and accordingly, cystic Gartners are no longer common either. One can surmise that chlorinated naphthalene would have has similar effects on other glandular structures in the vagina.

Bartholin's glands are referred to as the major vestibular glands.

Below, an image of the caudal portion of a cow's vagina, showing a cystic Bartholin's gland on the left and the openings of both left and right Gartner's ducts.

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Bartholin’s glands are anatomical analogs of bulbourethral glands (Cowper’s glands) in males. Their functions include lubrication and pheromone production. As seen above, they too, may become cystic, some cysts being as large as 10 cm in diameter. In one study of 102 cows, cysts were present in approximately 25% of the animals (higher than expected by this author from personal observations of slaughter plant specimens). Although a small number of cysts were present bilaterally in that study, most were unilateral and interestingly, mostly on the left side.

It is likely that cystic Bartholin's glands have little effect on reproductive efficiency unless (as has been reported) they become so large as to protrude from the vulva lips and become contaminated, predisposing the animal to vaginitis,

Selected references

Alam, M.G.S.and Rahman, A.  1979. Diseases of the genital tract of indigenous cows in Bangladesh Trop. Anita. Hlth Prod. 11:179-180

Bademkiran, S. et al. 2009. Unilateral Bartholin Gland Cyst in A Pregnant Heifer. F.Ü.Sağ.Bil.Vet.Derg. 23:61 - 63

Bland Sutton, J.  On the origon of certain cysts- ovarian, vaginal, sacral, lingual and tracheal. Journal of Anatomy and Physiology. Normal and pathological. 1886 Vol20. 439-442 (21 plates and several woodcuts).

Blazquez N.B. et al. 1987. Histology and histochemistry of the bovine reproductive tract
caudal to the cervix part I. The vestibule and associated glands. British Vet. J. 143:328-337

Blazquez N.B. et al. 1987. Histology and histochemistry of the bovine reproductive tract caudal to the cervix Part II. The vagina and associated structures. British Vet. J. 143: 337-343

Hatipoglu, F. 2002. An abattoir study of genital pathology in cows: II. Uterus, cervix and vagina
Revue. Med. Veterinaire t., 153: 93-100

Mägert, H. J. et al. 1995 cDNA sequence and expression pattern of the putative pheromone carrier aphrodisin. 92: 2091–2095.

Pande. M. et al 2011 Nabothian and endometrial cysts in a buffalo. Buffalo Bulletin. 30: 219-2

Pavone, L. M. et al 2009. Expression of Orexin A and Its Receptor 1 in the Vestibular Glands of the Cattle Genital Tract. The Anatomical Record 292:202–206

P.M. Summers, P.M. 1974. An abbatoir study of the genital pathology of cows
in Northern Australia. Australian Vet. J. 50: 403-406

Zaviacic, M.1. et al. 2000. Ultrastructure of the normal adult human female prostate gland (Skene's gland). Anat Embryol (Berl). 201:51-61

The bovine cervix, uterine body and horns

Keywords:  sperm, cervix, bovine

When specimens are submerged in water, anatomical features that are not normally visible, often become obvious. In this example, the appearance of the cervix is as it is usually presented on the left. It consists of three or four inter-digitating transverse folds between the cranial vagina and uterus. These folds are very firm and fibrous and must be negotiated when cattle are artificially inseminated. The image at right reveals that the cervix is far more complex than it appears to be at left.

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The reason for the complex arrangement of longitudinal cervical mucosal folds is not immediately evident.  However, in cattle, goats and humans, motile spermatozoa remain in the cervix longer than in other regions of the reproductive tract. This indicates that the cervix functions as a reservoir for spermatozoa. The cervix may also act as a filtration system, allowing spermatozoa to escape from seminal plasma and debris in the ejaculate. Certainly a filtration function is tempting to suggest in the case of another mammal, the Pilot whale. The extremely intricate and complex cervix of the Pilot whale can be seen in this LORI entry.

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The image above show the relationship between the cranial extent of the cervix, the uterine body and uterine horns. It is important to bear in mind the short length of the body (far shorter than in the mare) because when one makes a horn selection during embryo collection or insemination, the instrument must be turned immediately in the appropriate direction after entering the uterus. In mares, this is is not a consideration because the bifurcation lies 15 to 20 cm cranial to the internal cervical os in horses.

In this image it is interesting to note the presence of involuted caruncles. Note that the mere presence of caruncles does not indicate that this animal had once been pregnant because caruncles are present and easy to see, even in neonatal cattle.

Selected references:

Morton, D.B. and Glover T.D. 1974. Sperm transport in the female rabbit: the role of the cervix. J. Reprod. Fert. 38:131-138

Dobrowolski W and Hafez E.S.E. 1970. Transport and Distribution of Spermatozoa in the reproductive tract of the Cow. J Anim Sci. 31:940-943. This paper is a review article

Note: The author wishes to thank Dr Tammy Muirhead, Dept. of Anatomy, AVC. for her assistance in obtaining these images.

Abnormal fusion of the müllerian system

Keywords: cervix, paramesonephric, müllerian

During early fetal development in females, the müllerian ducts (also known as paramesonephric ducts) are permitted to develop because of the absence of anti mullerian hormone (AMH). This is due to the absence of testicles and the Sertoli cells therein; the source of AMH.

A note on terminology: Although the word testicle appears to be a diminutive term of the word testis (plural testes) in English, this is considered incorrect in scientific parlance. Nevertheless, it has become widely accepted as a colloquial term for the more correct "testis" in English. This is because the word testicle arose directly from French, where the word for a normal, single testis is "testicule".

Once mullerian tube development is underway, the most caudal sections of the tubules form to fuse the cranial segment of the vagina. When this does not occur, portions of the medial wall of the mullerian system can persist in the vagina into adulthood and cause dyspareunia or dystocia. In the author's experience, these remnants are most common in dogs and ruminants but they are well described in humans. These abnormalities should not be confused with persistence of the hymen which is the result of incomplete breakdown of the interface between the endoderm and ectoderm.

The degree of fusion of the müllerian tubes from caudal to cranial is a function of species in normal development. This can be seen in the image below. The degree of fusion in rabbits, opossums and other mammals with two cervixes is seen at left, progressing to greater and greater degrees of fusion with complete fusion in humans and higher primates at right.

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If fusion is abnormal and the species is bovine, a common manifestation is a double external os with immediate transition into a single cervical as shown here:

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The only abnormally un-fused portion of the mullerian system in the image above is the small transverse wall shown on the right side of the image, indicated by the light grey arrow. A similar case is shown below, where a finger was passed into the cervical os on one side, and out of the other. During calving, dystocia may arise if the extremities of the calf are separated by this defect. It can be transected without severe hemorrhage.

Notice the partial persistence of a hymen in this animal as well.

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In other cases, the degree of non fusion is more obvious, with two lumens progressing cranially for a short distance then fusing into a single canal. An example of this is shown below.

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In rare and extreme cases, there may be no fusion of the müllerian tubes at all (apart from fusion in the cranial vagina). This is a manifestation of the condition known as "uterus didelphys" from the Greek words di- (two) and didelphys (womb or uterus). The caudal portion of "uterus didelphys" in a cow is shown here:

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The author has had this image (above) for many years and cannot be sure of its origin. If you believe you know the author, please contact Dr Rob Lofstedt at lofstedt@upei.ca.

In a case of uterus didelphys with a double cervix as shown below, 

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a cow could be inseminated repeatedly through a cervix that does not connect with the side of the uterus where ovulation occurs, preventing conception on those occasions.

Note: In cattle, double cervixes are found at a level of less than 1% to about 15%, depending on the report and there is good evidence to suggest that it is a heritable condition; recessive with partial penetrance. Interestingly, this author encountered many cases of double cervixes in the Midwest US and came to think of the condition as quite common. However, after moving to New England then to Eastern Canada, he encountered relatively few cases. This substantiates the contention that it is heritable and more common in some gene pools than others. Although double cervixes have been reported in horses, they are rare. Indeed, this is also true for other domestic animals.

One should recall that the mullerian system in the embryo extends to the interface of fusion between the endo- and ectoderm. In the fetus, that interface becomes the hymen, either perforate or imperforate (persistent). Cranial to that point, all structures are mullerian in origin. Therefore remnants of the divided mullerian system can persist caudal to the cervix and cranial to the hymen. 

Persistence of the medial wall of the müllerian caudal to the cervix is not rare and can be a cause of dystocia with a fetal limb or head being trapped on one side of the structure and the rest of its body on the other. Remarkably, in some cases birth occurs normally. For example in the images below, the tract on the left was from a cow and that on the right from a heifer. Both animals had persistent medial walls of their müllerian systems. However, the uterus of the cow had undergone involution indicating that that she had calved despite the presence of the obstruction in her vagina. 

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The cervix of a postpartum cow

Keywords: cervix, involution, bovine, cow, endometrium, lochia

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The cervix of a cow seen at approximately one week after calving. This animal was euthanized because of a skeletal injury sustained during the birth of twin calves. Minor contusion and lacerations such as those seen here are common after parturition.

The bovine cervix closes down fairly rapidly after calving so that at this time (about a week after calving) it is difficult to introduce a hand into the uterus. However, the cervix remains open for at least 20 days to allow the discharge of lochia. This lochia becomes hemorrhagic at about 10 to 12 days when the remnants of the caruncles of pregnancy are sloughed to the level of the endometrial capilliary beds. It is also at about this time that the lochia begin to become purulent in nature as neutrophils enter the lumen from the capilliary beds. Therefore, if this photograph had been taken a few days later, it would not be unusual to find a hemorrhagic and somewhat purulent discharge being voided through the cervix.

Cervical mucus in a cow

Keywords: mucus, estrus, cervix, bovine, cow, detection, Spinnbarkheit, sodium chloride, NaCl, conductivity

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Mucus accumulation around the cervix of a cow during estrus. This mucus is being produced by the cervical glands around the external cervical os. Apart from its physiological value of lubricating the tract, its presence is a valuable management tool because it is discharged at the time of estrus in cows, facilitating estrus detection. 

The elasticity of the mucus is obvious in this image. The stretchiness of this mucus is sometimes referred to as Spinnbarkheit from the German word for "ability to spin (into thread) because it becomes thread-like when it is most able to stretch. In humans, this phenomenon is sometimes used to determine the optimal time for artificial insemination.

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Also, vaginal mucus from a cow in estrus, stretched between two glass slides:

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The concentration of salt (sodium chloride) increases during estrus therefore the ability of cervical mucus to conduct electricity also increases at that time. Instruments that measure conductivity of cervical mucus are available to assist one in determining the stage of the estrous cycle.