Pregnancy

PREGNANCY Pregnancy is referred to as the period of gestation and lasts a duration of 40 weeks. It is dated from the last menstrual period till the birth of the foetus. Following implantation, a hormone is secreted from the developing placenta which is called HCG (Human Chorionic Gonadotrophin). This hormone causes growth and development of the embryo as well as a series of changes in the mother which include nausea, vomiting, tiredness, etc. For the first 12 weeks of pregnancy, the Corpus Luteum continues to secrete Progesterone and oestrogen, which maintain the thickening of the endometrium. Meanwhile, the development of the different organ systems and specialisation of tissues takes place in the embryo, and the heart begins to beat. The placenta also matures and develops at this time. At 12 weeks the Corpus Luteum starts to degenerate and the placenta takes over the responsibility of secreting all the hormones necessary to maintain pregnancy. From 12 weeks until the end of pregnancy, the foetus grows in size and matures, while the uterus grows in size, becoming increasingly muscular. The foetus is particularly vulnerable during the first 12 weeks, as this is when the tissues and organs are laid down, and noxious substances which cross the placenta may cause detrimental damage. By approximately the 38th week of pregnancy, the foetus is upside down, with its head locked in the pelvic inlet. When the foetus is ready to be born, ( which usually coincides with the gradual degeneration of the placenta) the level of progesterone decreases rapidly such that it falls just below that of oestrogen. This intersection of hormones stimulates the pituitary gland to secrete Oxytocin. Oxytocin causes the uterus to contract. This starts a cycle of positive feedback, where the contraction of the uterus causes further secretion of Oxytocin, which in turn causes further contraction of the uterus such that the strength, frequency and duration of contractions is increased to such an extent, that the foetus is expelled from the uterus. LABOUR The onset of labour is marked by the first uterine contraction which is usually closely followed by the rupture of the amnion (bag of waters). The first stage of labour is where the frequent uterine contractions results in complete dilation of the cervix. The second stage of labour is the actual birth of the baby which usually takes place head first. The third stage, is the expulsion of the placenta which is called the afterbirth.