Fertilisation and implantation

Semen is deposited in the vagina during copulation; this is called insemination. The motile sperms swim through the cervix and the uterus until they reach the fallopian tube's ampullary region.

The ovum too is carried to the ampullary region, and fertilisation happens there. It can occur only when the ovum and the sperms arrive in this region together, which is why not every act of intercourse results in pregnancy.

When a sperm touches the zona pellucida, it triggers changes in the membrane that block entry of any more sperms, so only one sperm fertilises an ovum.

Acrosomal secretions help the sperm pass through the zona pellucida and plasma membrane into the ovum's cytoplasm.

Entry of the sperm triggers the secondary oocyte to complete meiosis II. This division, also unequal, produces a haploid ovum (ootid) and a second polar body. The haploid sperm and ovum nuclei then fuse, forming the diploid zygote.

Sex is fixed at fertilisation. The female gives only X-bearing ova, while sperms carry either X or Y (50 per cent each). An X sperm gives an XX (female) zygote and a Y sperm gives an XY (male) zygote, so the father's sperm decides the baby's sex.

The zygote divides by mitosis (cleavage) while moving through the isthmus towards the uterus, forming 2, 4, 8, 16 cells called blastomeres. The 8–16 cell stage is the morula.

The morula keeps dividing and becomes the blastocyst as it moves further into the uterus. Its cells are arranged into an outer layer, the trophoblast, and an inner cell mass attached to it.

The inner cell mass will give rise to the embryo, while the trophoblast becomes attached to the endometrium. The uterine cells then divide rapidly and cover the blastocyst, so it becomes embedded in the endometrium; this is implantation, and it leads to pregnancy.