4 weeks pregnant: Changes for baby

The blastocyst (the medical name for the bundle of cells which is your baby at this stage) becomes more firmly implanted in the uterus this week. By about the 12^th day after fertilisation (day 27 if fertilisation occurred on day 15) it is fully implanted into the wall of your uterus (womb), and beginning to form connections to the circulatory system of the uterus. Tiny blood vessels called chorionic villi begin to grow and connect the blastocyst to the uterus. They latch onto the blastocyst, towards the end of this week and will eventually cover the entire placenta. These blood vessels will work to supply nutrients from your body to your baby’s via the placenta, for the remainder of your pregnancy.

How big is your baby now?

By the end of the second week (day 28 of your pregnancy or 14 days after conception) your baby is still tiny, approximately 1mm long! To put this in perspective, this is the thickness of about 8 sheets of paper.

How is your baby growing?

While the cells which will form the placenta are busy producing hCG, the collection of cells that will grow into your baby, also begin to change. A small space is formed between the cells with a single layer of cells above it- these go on to become the membranes which are the structures that connect the embryo to your placenta. Below the space a plate of cells two layers thick, called the bilaminar disc, forms. It is from the bilaminar disc that each of the different types of tissue, for each of the different structures of your baby’s body (e.g. the nervous system, the eyes) begin to differentiate next week, when you are 5 weeks pregnant.

The sac which holds the amniotic fluid also forms this week. Amniotic fluid is a transparent fluid, slightly yellow in colour, in which your baby floats throughout your pregnancy. It is held in the amniotic sac. Amniotic fluid helps protect your baby from injury and maintain a constant temperature in your womb. Later in your pregnancy when your baby starts to move around, the amniotic fluid facilitates movement, which in turn helps to ensure your baby’s bones develop correctly. It also plays an important role in helping your baby’s lungs develop.

References

More information on the 4^th week of pregnancy

For more information on the fourth week of pregnancy, see 4 weeks pregnant.

For more information on the pregnancy symptoms which occur before a woman even knows she’s pregnant, see Changes for mum this week.

For more information on doctor appointments and other lifestyle changes to adopt, see Pregnancy health and lifestyle information this week.

For more information on the key points of fourth week of pregnancy, see Key points this week.