Explain how pregnancy is sustained until parturition.

In the events of pregnancy

Implantation

Following fertilisation, the zygote divides (cleavage) mitotically until a hollow ball of cells, the blastocyst is produced. It takes three days to reach the uterus and a further three or four days to become implanted in the lining of the uterus. The outer layer of the blastocyst, called the trophoblast, develops into embryonic membranes, the chorion and amnion.

The chorion develops villi which grow into the surrounding uterine tissue from which they absorb nutrients. These will form part of the placenta which is connected to the foetus by the umbilical cord.

The amnion develops as a membrane around the foetus and encloses the amniotic fluids, a watery liquid which protects the foetus by cushioning it from physical damage.

The corpus luteum persist due to the secretion a hormone called Human Chorionic Gonadotropin (HCG) by the placenta. This hormone signals to the mother’s body that an embryo is present in the uterus.

The corpus luteum continues to secrete progesterone which coupled with small but stead secretion of oestrogen, maintain the continued development of the uterus and prevents menstruation.

After the first three or four months of pregnancy, the corpus luteum begins to regress and the job of secreting oestrogen and progesterone is take over by the placenta.

In this way, the endometrium is maintained in a suitable state throughout pregnancy.

The hormonal control of birth

Towards the end of pregnancy, the levels of oestrogen in blood rises while that of progesterone falls. It has been suggested that this plays some part in bringing about birth. Indeed, oestrogen promotes uterine contraction whereas progesterone doesn’t.

But the most direct cause of birth in another hormone, oxytocin secreted by the posterior lobe of pituitary gland that cause uterine muscle contraction.

Oestrogen and progesterone are also responsible for the growth of the mammary gland in preparation for milk production (lactation). After birth, milk flow is initiated by a hormone called prolactin secreted by the anterior lobe of the pituitary gland.