What is the first trimester?

A full-term pregnancy lasts around 9 months, and most people (including your doctors and midwives) will break this up into 3 ‘trimesters’. Although you are pregnant from the moment of conception – when a male sperm fertilises your ovum (egg) – the first trimester of pregnancy is counted from the first day of your last period through to week 12. This is because most women who conceive naturally won’t know the date of conception.

Trimesters are a helpful way to think about pregnancy because the changes that happen to you and your baby fall into the 3 broad categories of early, middle and late pregnancy, called first, second and third trimesters.

What happens to your body?

For some women, the first trimester is characterised by nausea (often called ‘morning sickness’, although it can occur at any time of day). But remember that every pregnancy is different and while some women have food cravings, others experience food aversions, and some have no change in appetite at all.

Other changes in the first trimester include changes to your breasts as they become tender, larger and heavier, while your uterus will grow and put pressure on your bladder so that you need to urinate more often.

What happens to the baby?

Through the first trimester, your baby goes from being a fertilised ovum to a fetus of about 6cm in length at 12 weeks. By the end of the first trimester, your baby’s heart is starting to beat, and the brain, stomach and intestines are developing. There are little bumps known as ‘buds’ where arms and legs are starting to grow.