BA (Hons) MRCOG
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.
Your body also undergoes an extraordinary amount of changes, most of which you will not be aware of. Despite this, by the end of Week 13, you probably still do not look pregnant, and the only change that might be noticeable to others is that your breasts have become larger. Initially, your pregnancy is supported by hormones alone until the placenta takes over towards the end of the trimester.
Your baby starts to develop as soon as the egg is fertilised by the sperm (see picture), that is, in Week 3 of your pregnancy (see below). Fertilisation is described in detail in Chapter 1, p.10. On its journey towards the uterus, the egg (blastocyst) has been rapidly dividing into more and more cells, which become specialised: the outer, trophoblast cells develop into the placenta, while the inner cells are already programmed to form three different layers, the ‘germ’ layers, which will grow
into the various parts of your baby:
Six days after fertilisation, the blastocyst attaches itself to the endometrium (lining of the uterus). This is called implantation.
If you have become pregnant following assisted conception, the date that you had your embryo transfer (whether frozen or otherwise) is counted as Day 14 of your pregnancy, and all your pregnancy dates thereafter will be calculated on that basis.