Fertilization and Pregnancy

 After semen is ejaculated into the female reproductive tract, spermatozoa are transported up to the oviducts fairly rapidly mainly by the action of cilia and muscular contractions in the female reproductive tract. The spermatozoa must arrive at the oviduct before the ova do. They must undergo capacitation in the oviduct to increase their ability to fertilize ova.

Meeting of the ova with particular spermatozoa is entirely by chance. Once the head of a spermatozoon enters the ovum, others are blocked from entering. The nuclei of the spermatozoon and ovum come together and the diploid chromosome number is restored. The fertilized ovum is called a zygote.

The zygote begins a series of rapid cell divisions called cleavage. The divisions take place so rapidly that by the time the zygote is ready to implant in the lining of the uterus in a few days, it has formed into a hollow ball of cells (the blastocyst) not much larger than the original ovum.

As the implanted zygote develops, it forms a placenta around itself that allows it to draw nutrition from the lining of the uterus and send wastes back. The placenta is a multilayered membranous sac that surrounds the developing fetus and suspends it in fluid. The fetus is connected to the placenta by the umbilical cord.

Early pregnancy in a cat.

Preserved pregnant bovine uterus showing placentomes.

Preserved calf fetus and uterus showing umbilical cord.

The gestation period is the period of pregnancy. Its length is fairly consistent within a species.

Parturition is the birth process. It consists of three stages.

Involution of the uterus - shrinkage of the uterus after parturition back to its non-pregnant size. It helps control post-birthing hemorrhage from sites where the placenta disattached.