The first trimester begins on the first day of your last menstrual period and is considered to finish at the end of the thirteenth week. It is a crucial time in your pregnancy, during which the baby’s major organs are formed and external physical features develop. Initially, your pregnancy is supported by hormones alone until the placenta takes over towards the end of the trimester.

Weeks 1 - 4

The implanted embryo has started to produce hormones that may register on a home pregnancy test, yielding a positive result. For some women, the test will not register as positive for another week.

There are three main pregnancy hormones: oestrogen, progesterone and beta human chorionic gonadotropin (BhCG), each of which has a specific function in maintaining your pregnancy.

Early pregnancy

The higher oestrogen levels that occur in pregnancy help to thicken the endometrium, ensuring that the developing embryo remains safely embedded and able to obtain all the nutrients it needs to continue to grow. Oestrogen also stimulates the growth of milk glands in your breasts, which usually causes them to feel tender in these early weeks. This tenderness should disappear by the start of the second trimester.

BhCG is a hormone that occurs almost exclusively during pregnancy. It is secreted by the cells of the implanting egg and later, at a lower level, by the placenta. Like oestrogen, it also helps to keep the embryo embedded in the endometrium. Secretion of the hormone rises sharply in the first few weeks of pregnancy and peaks at around weeks 8–10.

This is an edited extract from One Born Every Minute: Expecting a Baby? by Dr Penelope Law (Quadrille, £25).

Text © 2013 Dr Penelope Law