Fertilization and development of the embryo
Fertilisation occurs inside the mother’s body, in the uterine tube: a sperm cell joins an egg cell, forming a zygote. The zygote divides and becomes a multicellular embryo; in about a week the embryo settles into the uterine wall. Membranes and a fluid-filled sac that protect it form. The placenta forms from the embryo’s villi and the uterine lining: through it oxygen and nutrients pass from mother’s blood to the fetus, and carbon dioxide and wastes are carried away; maternal and fetal blood do not mix. After about 8 weeks the embryo is called a fetus; organs form during this period and then grow and mature; the heart begins to beat as early as the first month. Pregnancy lasts about 9 months (270–280 days). During birth the uterine muscles contract strongly (with hormones including a pituitary hormone), the baby is born with the help of a doctor or midwife; the newborn’s first cry shows it has begun to breathe with its lungs, and the umbilical cord is cut. During pregnancy medical supervision, proper nutrition, vitamins, vaccination and complete avoidance of smoking and alcohol are important for the mother: these harm the fetus’s development.
Stage ribbon: write “fertilisation → zygote → embryo → fetus → birth” on 5 cards and add one fact to each (e.g. “placenta – exchange organ”). Arrange in order and explain to a friend.