Pregnancy extends from fertilization to delivery, normally about 40 weeks from the last menstrual period (38 weeks from ovulation).
Fertilization occurs in the ampullary portion of the fallopian tube, usually within a day of ovulation.
Gamete Transport, Fertilization and Cleavage
Capacitation: changes sperm undergo in the female genital tract (altered tail beat, plasma membrane change) that make them capable of fertilizing the egg.
Fertilization has four steps:
Fusion of sperm with egg.
Acrosome reaction with sperm penetration.
Polyspermy block.
Zygote formation.
Acrosome reaction releases acrosomal enzymes, including acrosin, that digest the zona pellucida so the sperm can penetrate to the egg membrane.
Polyspermy block: sperm-egg fusion first changes the egg's membrane potential, then egg-released enzymes harden the zona pellucida, both blocking further sperm entry.
The zygote undergoes mitotic cleavage in the fallopian tube:
blastomere by 24 hours
morula (16 to 32 cells) by 96 hours
entering the uterus by day 4
Sperm transport through the cervix is favoured by estrogen-induced thinning and parallel orientation of cervical mucus; of the millions deposited, only about 50 to 100 reach the fallopian tube.
Monozygotic twins arise when dividing blastomere cells separate into two independent cell masses; dizygotic twins arise from two eggs ovulated and fertilized simultaneously.