Fertilisation, pregnancy and birth

Fertilisation. During mating, sperms are released into the vagina and swim up through the cervix and uterus into the oviduct. The ovum, released at ovulation, is carried into the oviduct by the fimbriae. Fertilisation happens in the ampullary region of the oviduct, so the ovum and sperms must arrive there together.

A sperm touches the ovum's outer layer, the zona pellucida. This changes the zona so that no other sperm can enter. The sperm's acrosome enzymes help it get in, the secondary oocyte finishes meiosis II (giving off a second polar body), and the sperm and egg nuclei fuse to form a diploid zygote.

Who decides the baby's sex? Every ovum carries an X chromosome. Half the sperms carry X and half carry Y. An X sperm gives an XX zygote, a girl; a Y sperm gives an XY zygote, a boy. So it is the father's sperm that decides the sex of the child.

From zygote to implantation. As it moves down the oviduct, the zygote divides by mitosis (cleavage) into 2, 4, 8, 16 cells, the blastomeres. An embryo of 8-16 blastomeres is a morula. It becomes a hollow ball, the blastocyst, with an outer layer, the trophoblast, and an inner cell mass. The trophoblast attaches to the endometrium, and the blastocyst becomes embedded in the uterine wall: implantation. This starts pregnancy. The inner cell mass will become the embryo itself.

The placenta. Finger-like chorionic villi from the trophoblast and the uterine tissue together form the placenta, which connects the embryo to the mother through the umbilical cord. Through it the embryo receives oxygen and food and gets rid of carbon dioxide and wastes. The placenta also makes hormones: hCG, hPL, estrogens and progestogens. hCG, hPL and relaxin are made only during pregnancy.

Development. The heart forms by the end of the first month; limbs and fingers by the end of the second; most organs by the end of the first trimester (three months). Human pregnancy lasts about 9 months.

Birth (parturition). At full term, the fully grown fetus and the placenta send signals that start mild contractions: the foetal ejection reflex. This makes the mother's pituitary release oxytocin, which causes strong contractions of the uterus muscle, and the baby is pushed out through the birth canal.

Lactation. The mammary glands make milk after birth. The milk of the first few days, colostrum, contains antibodies that protect the newborn, so breastfeeding is strongly recommended.