Fertilisation, implantation and development of the embryo
Sperm and ovum fuse in the fallopian tube to form a zygote. It divides, implants in the uterus lining, and the placenta then feeds the growing baby.
From zygote to baby
The zygote divides by mitosis while it moves down the fallopian tube to the uterus.
How fertilisation happens
- 1During mating (sexual intercourse), semen is released into the vagina.
- 2Sperms swim through the cervix and the uterus into the fallopian tube.
- 3The ovum, released at ovulation, is in the fallopian tube.
- 4Enzymes from the acrosome help one sperm break through the coverings of the ovum.
- 5★The nuclei of the sperm (n) and the ovum (n) fuse → zygote (2n).
- 6Once one sperm enters, the ovum blocks all the others.
★ key step
Jobs of the placenta
- Passes food and oxygen from the mother's blood to the foetus.
- Takes carbon dioxide and wastes from the foetus to the mother's blood, to be removed.
- Makes hormones, like progesterone, that keep the pregnancy going.
- Keeps out many germs, though alcohol, nicotine, some medicines and some germs can still cross it.
- The umbilical cord joins the foetus to the placenta. The blood of mother and baby does not mix directly.
- A fluid-filled sac, the amnion, cushions the baby against shocks.
Birth and after
Pregnancy lasts about 280 days (around 9 months). This is the gestation period. At the end, the hormone oxytocin makes the muscles of the uterus contract strongly. The baby is pushed out through the cervix and vagina. This is parturition (childbirth). After birth, prolactin makes the breasts produce milk, and oxytocin helps release it. The first milk, colostrum, is rich in antibodies that protect the newborn.
Pregnant women get iron and folic acid tablets and regular check-ups at government health centres; pulses, greens, milk and eggs help the baby grow. Breastfeeding from the first hour gives the baby colostrum, full of antibodies.
2 marks: “What is implantation?” The sinking of the blastocyst into the endometrium of the uterus, about 6 to 7 days after fertilisation.
2 marks: “Write any two functions of the placenta.” Passes food and oxygen to the foetus; removes carbon dioxide and wastes; makes hormones (any two).
4 marks: “Describe the events from fertilisation to implantation.” Sperm meets ovum in the fallopian tube; acrosome enzymes; zygote (2n); cleavage → morula → blastocyst; moves to the uterus; implants in the endometrium about 6 to 7 days after fertilisation; placenta forms.
Zygote, Morula, Blastocyst, Implantation: 'Zebras Make Big Impressions'.