Reproduction and genetics, Year 9: The menstrual cycle and pregnancy

Menstruation (a period)

提示The whole monthly cycle is named after this event.

为什么Each month the lining of the uterus thickens in case a fertilised egg arrives. If none does, the lining is no longer needed, so it breaks down and leaves the body with some blood. A period usually lasts about five days.

28

提示It is a little shorter than most calendar months.

为什么A cycle is counted from the first day of one period to the day before the next. Twenty-eight days is the average; anything from 21 to 35 days is normal, and it varies from person to person.

shed; thickens

提示Old out first, then a fresh build-up.

为什么Day 1 of the cycle is the first day of the period. Once the old lining has gone, a new one grows, rich in blood vessels. About halfway through the cycle an egg is released from an ovary, and the thick lining is ready in case that egg is fertilised.

To receive and nourish a fertilised egg

提示A tiny embryo needs somewhere soft and rich in blood to settle.

为什么If an egg is fertilised, the ball of cells it becomes sinks into the uterus lining, which supplies it with food and oxygen until the placenta has formed. Preparing this lining is the whole purpose of the monthly cycle.

The uterus lining is kept, because the embryo is growing in it

提示Ask what the usual monthly clear-out would do to what has just settled there.

为什么The lining is only shed when no fertilised egg has arrived. In pregnancy it stays in place and develops further, so the cycle pauses until after the baby is born. A missed period is therefore often the first sign of pregnancy.

Gestation

提示The same word is used for an idea that is slowly taking shape.

为什么During gestation a single fertilised cell divides and develops into an embryo and then a fetus with working organs. Mammals differ widely: gestation takes about three weeks in a mouse and nearly two years in an elephant.

40

提示It is a round number, a little over nine months.

为什么Doctors count from the last period because that date is known, which makes the total about 40 weeks, or 280 days. A baby born between 37 and 42 weeks has arrived at the normal time.

No — substances diffuse across a thin barrier between them

提示A baby can have a different blood group from its mother.

为什么The two blood supplies stay separate. Oxygen and dissolved food diffuse from the mother's blood into the baby's, and carbon dioxide and other wastes diffuse the other way. Keeping the bloods apart protects the fetus from the mother's higher blood pressure and from many infections.

oxygen; carbon dioxide

提示The same two gases your own lungs swap with the air.

为什么A fetus cannot breathe or eat, so everything it needs arrives through the placenta and everything it must get rid of leaves the same way. The mother then removes the wastes through her own lungs and kidneys.

The harmful substances cross the placenta and reach the fetus

提示Whatever is dissolved in the mother's blood is brought very close to the baby's.

为什么Alcohol passes from the mother's blood to the baby, whose liver cannot yet deal with it, and can damage its development. Smoking cuts the oxygen reaching the fetus, and babies of smokers are on average born lighter. A mother's lifestyle directly affects her unborn child.