Editing Calf (animal) (section)

=== Moment of calving ===

==== Contractions ====
The uterine muscle contractions, or [[myometrium]], facilitate the calf's progression through the pelvic canal during calving. These contractions, known as "colic," begin approximately six hours before delivery. Initially, they are infrequent (every seven minutes) and brief, lasting only a few seconds. As calving progresses, the contractions become closer together and longer. Near the critical moment, they last about one minute and are spaced similarly.<ref name=":23"/> These repeated contractions gradually move the calf through the pelvic canal, allowing it to pass the cervix and reach the vulva. Internal tension causes the allantoic sac to rupture, releasing the "first waters."<ref name=":33"/>

==== Fetus expulsion ====
Shortly after the rupture of the allantoic sac, the amniotic sac appears at the vulva. This sac also tears under the cow's expulsive efforts.<ref group="note">Sometimes, the amniotic sac does not rupture, which ultimately results in the death of the calf, trapped inside this sac and unable to breathe.</ref> The head and front legs are soon present at the vulva, which gradually dilates to allow their passage. This stage of calving is the most painful for the cow, requiring significant expulsive effort. Once the chest has passed through the pelvic canal, only a few more contractions are needed to expel the entire calf, followed by the remaining fluids from the amniotic and allantoic sacs.<ref name=":4">{{Cite journal |last1=Derivaux |first1=J |last2=Ectors |first2=F |date=1981 |title=Physiopathologie de la gestation et obstétrique vétérinaire |trans-title=Pathophysiology of gestation and veterinary obstetrics |url=https://www.persee.fr/doc/bavf_0001-4192_1981_num_134_1_7540 |journal=Bulletin de l'Académie Vétérinaire de France |language=fr |volume=134-1 |pages=53–55}}</ref>

The umbilical cord breaks only after the fetus has completely exited the vulva.

Calving is relatively slow in cows, especially first-time mothers. It can last between 30 minutes and three hours. The separation of the maternal cotyledons from the fetal cotyledons occurs slowly, allowing circulatory exchanges between the mother and calf to continue until the fetus is expelled. This explains why longer deliveries in cows do not significantly reduce the calf's chances of survival.<ref name=":4" />

==== Calf presentation ====
Typically, the calf is positioned in the "dorso-sacral" posture, with the forelegs and head emerging first through the pelvic canal. However, in 5% of cases, the hind legs present first. This variation slightly prolongs calving and reduces the calf's survival chances, as the umbilical cord may break prematurely, potentially causing asphyxiation.<ref name=":33"/>

==== Twin births ====
[[File:Veauxjumeaux.jpg|left|thumb|Twin Charolais calves]]
Multiple births are relatively uncommon in cattle, with the natural twin birth rate estimated at 3%. Twin pregnancies are generally associated with a reduction of 3 to 6 days in the gestation period. However, twin gestations can also have specific consequences on the calving process. While the risk of a size mismatch between the fetus and the mother's pelvis is lower, there is a higher probability of fetal malposition or simultaneous presentation of both fetuses. Furthermore, excessive uterine dilation caused by carrying twins can lead to uterine inertia and insufficient contractions. In twin pregnancies, it is common for one fetus to be in an anterior presentation while the other is in a posterior presentation.<ref name=":52">{{Cite book |last1=Noakes |first1=D |title=Arthur's Veterinary reproduction and obstetrics 8e volume |last2=Parkinson |first2=T.J |last3=Englang |first3=G.C.W |date=2001 |publisher=W.B.Saunders}}</ref> Twin calves are often weaker and more prone to neonatal conditions such as anoxia.<ref name=":13"/> The rate of stillbirths is also higher in twin pregnancies.<ref name=":52"/>

In cases of fraternal twins of different sexes, testosterone production by the male fetus can hinder the normal development of the female fetus's reproductive organs, leading to an increased incidence of sterility in heifers from such pregnancies. This issue is particularly pronounced when both fetuses develop in the same uterine horn, as it increases the likelihood of placental connections. The occurrence of [[freemartin]]ism is estimated at 90 to 95% in twin pregnancies involving fetuses of different sexes. These mixed-sex twin pregnancies account for 42 to 46% of all twin cases.<ref>{{Cite book |last=Educagri |url=https://books.google.com/books?id=Rc6C73CZZz0C&dq=bovin+jumeau+st%C3%A9rilit%C3%A9&pg=PA115 |title=Reproduction des animaux d'élevage |date=2005 |publisher=Educagri Editions |isbn=978-2-84444-410-3 |language=fr |trans-title=Reproduction of farm animals, by Educagri}}</ref>