Expansion
Fertilisation in the ampulla, implantation about 6 to 10 days later
Expansion
- Capacitation: sperm undergo functional maturation in the female tract over several hours, becoming able to fertilise
- Fertilisation in the ampulla of the fallopian tube, within about 24 hours of ovulation
- Acrosome reaction: enzymes digest the zona pellucida
- Cortical reaction after the first sperm enters, hardening the zona and preventing polyspermy
- Completion of the second meiotic division of the oocyte
- Cleavage as the conceptus travels down the tube: morula by day 3 to 4, blastocyst by day 5
- Implantation in the endometrium at about day 6 to 10 after fertilisation
- Syncytiotrophoblast produces hCG, rescuing the corpus luteum
The oocyte survives about 24 hours and sperm about 3 to 5 days in the female tract, giving a fertile window of roughly 6 days ending on the day of ovulation.
Ectopic pregnancy follows from step 2 and 7: anything delaying tubal transport, such as previous pelvic inflammatory disease, tubal surgery or an intrauterine device, allows implantation in the tube, most often the ampulla.