Pregnancy lasts a standard 40 weeks, counted from the first day of the last menstrual period, and is divided into three trimesters. Each one brings different changes in the baby’s development and in the mother-to-be’s body. This guide takes you week by week through the most important stages, so you know what to expect and when to plan key tests.
How are the weeks of pregnancy counted?
Gestational age is given in weeks and days, for example 12+3 means 12 weeks and 3 days. Counting starts from the first day of the last menstrual period, not from conception. For this reason, in the first two weeks of the formal pregnancy a woman is not yet pregnant in the biological sense — ovulation and conception occur only around week 2.
First trimester: weeks 1–13
This is the period of the most intense embryonic development and the formation of organs.
- Weeks 1–4: fertilisation, cell division and implantation of the embryo in the uterus. The first sign is often a missed period.
- Weeks 5–8: the heart starts beating, the buds of the limbs and brain form. Typical complaints appear: nausea, drowsiness, breast tenderness.
- Weeks 9–13: the embryo becomes a foetus and already has outlined facial features. This is the time for the ultrasound between week 11 and 14 and the so-called combined (double) test.
Second trimester: weeks 14–27
For many women this is the most pleasant period of pregnancy — nausea usually subsides and the bump becomes visible.
- Weeks 14–18: the baby grows intensively, the first movements appear, though they are not yet felt.
- Weeks 18–22: the so-called mid-pregnancy (anatomy) ultrasound, assessing the structure of the organs. It is often possible to find out the baby’s sex.
- Weeks 24–27: hearing develops and the baby responds to sounds. A glucose tolerance test is carried out to screen for gestational diabetes.
Third trimester: weeks 28–40
This is the final stage, during which the baby gains weight and prepares for birth.
- Weeks 28–32: the lungs mature and the baby opens its eyes. More frequent check-ups and another ultrasound.
- Weeks 33–37: the baby usually settles head down, getting ready for birth.
- Weeks 38–40: the pregnancy is full-term. Birth can happen at any moment during this period.
When is a birth full-term?
A birth is considered full-term between the 37th and 42nd week of pregnancy. A baby born earlier is described as premature, and a pregnancy lasting more than 42 weeks is described as post-term and requiring closer observation.
The key tests in brief
- First-trimester ultrasound (weeks 11–14) together with biochemical screening.
- Mid-pregnancy ultrasound (weeks 18–22) assessing foetal anatomy.
- Glucose tolerance test (weeks 24–28).
- Third-trimester ultrasound and check-ups before birth.
This calendar is for information only and does not replace medical consultation. The individual schedule of visits and tests is always set by the doctor or midwife looking after the pregnancy.
Check which week you are in and when you will give birth: Due date calculator →