Introduction
Pregnancy begins when a spermatozoon fertilizes the secondary oocyte in the ampulla of the tube, usually within 12–24 hours of ovulation. Sperm survive 3–5 days in the female tract.
Fertilization
- Capacitation of sperm in the female tract and the acrosome reaction at the zona pellucida.
- Fusion of one sperm with the oocyte; the cortical (zona) reaction blocks polyspermy.
- The oocyte completes meiosis II; the male and female pronuclei fuse into a diploid zygote (46 chromosomes). Sex is set by the sperm (X or Y).
Cleavage and implantation
The endometrium is receptive only in a short implantation window (about days 20–24 of a 28-day cycle), prepared by progesterone; after implantation it becomes the decidua (basalis, capsularis, parietalis).
Embryonic and fetal development
Landmarks by gestational age (from the LMP)
- 6 weeks: embryo with heartbeat on transvaginal ultrasound.
- 12 weeks: all organs formed, crown–rump length about 5–6 cm.
- 20 weeks: quickening, fetus about 300 g.
- 24 weeks: limit of viability, about 500–600 g.
- 28 weeks: about 1,000–1,100 g; 40 weeks: about 3,300–3,500 g, 50 cm.
Gestational age is counted from the last menstrual period, two weeks more than the age since fertilization: 40 weeks of pregnancy are 38 weeks of embryo-fetal life.