10.7 Fertilization, Pregnancy and Development

Notes

• Fertilization: a sperm fuses with a secondary oocyte in the uterine tube, forming a zygote with 46 chromosomes (23 from each parent). Gametes are haploid (23 chromosomes), made by meiosis.
• The zygote divides (cleavage) into a ball of cells (morula) and then a blastocyst, which implants in the endometrium about 6 to 7 days after fertilization. The placenta forms from embryonic and maternal tissue and exchanges nutrients, gases and wastes; the mother's and baby's blood do not mix. The placenta also makes hCG, which maintains the corpus luteum (and is the hormone detected in pregnancy tests), plus estrogen and progesterone.
• Terms: embryo (weeks 2-8), fetus (week 9 to birth). Pregnancy lasts about 38 weeks from fertilization (40 from the last period). Organs form mostly in the first trimester, which is when the embryo is most sensitive to harmful substances (alcohol, some drugs, infections).
• Birth: positive feedback with oxytocin. Stages of labor: dilation, expulsion, placental stage.
• Lactation: prolactin makes milk; oxytocin ejects it.