Fertilisation, pregnancy, and development
From a single fertilised cell, a human develops through about 38 weeks of pregnancy into a newborn of roughly 3.5 kg.
Fertilisation
Of the millions of sperm deposited, only a few hundred reach the egg in the uterine tube. Acrosome enzymes help one sperm penetrate the egg's outer layers; the egg then changes its surface to block other sperm. The egg completes meiosis, and the nuclei fuse to form a diploid zygote. The sperm's X or Y chromosome determines chromosomal sex.
Early development and implantation
The zygote divides as it travels down the tube, forming a ball of cells and then a blastocyst, a hollow sphere with an inner cell mass that becomes the embryo. About 6-7 days after fertilisation, the blastocyst implants in the endometrium. Implantation outside the uterus, usually in the tube, is an ectopic pregnancy, a medical emergency.
Pregnancy hormones
The implanted embryo's outer cells secrete human chorionic gonadotropin (hCG), which keeps the corpus luteum making progesterone so the lining is not shed. Pregnancy tests detect hCG in urine. By about 3 months, the placenta takes over making progesterone and oestrogen.
The placenta
The placenta, formed from embryonic and maternal tissue, exchanges oxygen, nutrients, and wastes between maternal and fetal blood without mixing them, connected to the fetus by the umbilical cord. Some substances, including alcohol, nicotine, certain drugs, and some viruses, cross the placenta and can harm development.
Embryonic and fetal stages
The embryonic period, weeks 3-8, is when all major organs form (organogenesis), so it is the period most sensitive to harmful substances. The heart begins beating around week 4-5 of development. From week 9 the developing human is called a fetus, and growth and maturation dominate. Babies born before 37 weeks are premature.
Labour and birth
Labour begins with regular uterine contractions. Oxytocin strengthens contractions, which push the baby against the cervix, triggering more oxytocin: positive feedback. Labour has three stages: dilation of the cervix to about 10 cm, delivery of the baby, and delivery of the placenta (afterbirth).
Changes at birth
Before birth, fetal blood largely bypasses the lungs through shunts such as the foramen ovale (a hole between the atria) and the ductus arteriosus. With the first breaths, lung pressure drops, blood flows through the lungs, and these shunts close over hours to days. Failure to close causes some congenital heart conditions.
Inheritance and twins
Identical (monozygotic) twins come from one zygote that splits, so they share the same DNA. Fraternal (dizygotic) twins come from two eggs fertilised by two sperm and are as related as ordinary siblings. Twin studies help researchers separate genetic and environmental influences on traits.
Key takeaways
Fertilisation usually happens in the uterine tube, forming a zygote. The blastocyst implants about a week later. hCG maintains the corpus luteum and is detected by pregnancy tests; the placenta later takes over hormone production and exchanges materials. Organs form in weeks 3-8, the most sensitive period. Labour uses oxytocin positive feedback. Fetal shunts close after birth.
Flashcard
Fertilised egg
Check your understanding
Why is exposure to harmful substances most dangerous during weeks 3 to 8 of development?