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Pregnancy week by week

From the moment the sperm reaches the egg, you are pregnant. Without even being aware of it, a miracle has played out in your belly! In a short period of time the fertilised egg grows into a foetus that already has its major organs and systems. About 40 weeks after the first day of your last period, your baby will be born.

The first trimester – week 1 to week 12

In the first weeks of your pregnancy you often don’t know that you’re pregnant. The fertilised egg will quickly multiply until it’s a ball of cells. After implanting in the uterine wall the cells will develop into an embryo and placenta. The placenta is responsible for supplying nutrients and oxygen to the baby. The heart is the first organ to start working as it circulates blood. The beginnings of the brain and spinal cord are also formed in the first weeks. After about 10 weeks other major organs begin functioning, such as the intestines, brain and liver. Bones and cartilage are made. Even the sex organs are already formed, although they are not yet visible on the ultrasound. At the end of the first trimester the baby has all major organs and systems which will further ripen and grow with development. In the first trimester the embryo will grow into a foetus measuring 7cm and weighing 33g.

The second trimester – week 13 to week 26

In the second trimester the foetus grows from 7cm and 33g to 37cm and 940g. It’s not just growth that is rapid but development is moving at a fast pace. The foetus is training muscles and practices breathing, sucking and swallowing, skills that will be needed to survive outside the womb. The development of the nervous system leads to an improvement of the various senses. From inside the womb the foetus can now hear you and will even eventually recognise your voice. His eyes can detect the light that shines through your belly. Halfway through the trimester you will feel him kick for the first time! He may have the hiccups, yawn, suck his thumb and exhibit behaviours that are more and more like a baby. While the baby looks fully developed, he still needs to do a lot of growing and his organs are not yet ready for life outside the womb.

The third trimester – week 27 to delivery

In the third trimester the baby grows from 37cm and 940g to about 50cm and 3400g. While his length this trimester will not even double, he will become four times as heavy! The majority of weight gain is due to fat, which will keep him warm once he is born. At the start of the third trimester your baby’s eyes will open and he will soon be able to blink. He doesn’t just look like a baby but also behaves like one, making faces, swallows, hiccupping, moving his arms and legs, sucking on fingers and moving his head. His sensory abilities continue to improve and he even already dreams while he sleeps. His brain is quickly developing and will continue to do so after he is born. Other organs like the liver and kidneys are fully developed. Cartilage continues to be converted to bone tissue. His skull, which will stay somewhat softer, is composed of ‘loose plates’ that are able to overlap during the birth. As there is little room left in your uterus for the baby by the end pregnancy, he will choose a position he feels comfortable in. Most babies choose a head down position. After 37 weeks your baby is full term and labour can begin. However, your baby may also choose to stay in until 42 weeks. No matter what, the trimester will end with the birth of your baby!


In de zwangerschapskalender op de site volg je de ontwikkeling van jezelf en de baby in je buik op de voet. Je leest hier wat er van week tot week gebeurt en krijgt er handige tips.

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