Pregnancy Week 21 – The Baby’s Progress

By pregnancy week 21 your baby already has a high number of red blood cells and now begins developing white blood cells as part of their own immune system development. The bone marrow is now mature enough to take over the job of making it’s own red blood cells, a job that was previously carried out by the liver and spleen.


Pregnancy at 21 weeks – diagram of a cross section of bone marrow

The bone marrow is the spongy bit in the middle of the bone. As well as the red blood cells, it also produces plasma cells (the yellow, clotting fluid that red blood cells are suspended in). One of the important jobs of plasma is to create antibodies that help repair damaged cells and fight infection. Once the cells in the bone marrow have matured they then get released into blood stream with passes through the circulatory system delivering oxygen, essential nutrients and natural defense to the baby.

Most women will, by now, know the sex of their child, but some, however, prefer to keep on guessing, all adding further excitement to the day. One of the greatest pregnancy myths is being able to tell the gender of your baby by how high, low or round your fundus (pregnancy bump) is.
There are many factors that attribute to the size and shape of your belly and none of them are relative to whether you are carrying a boy or a girl. The lay of your fundus has to do with which way the baby is positioned in the uterus, how much weight you have gained during your pregnancy, how much your muscles have stretched (including previous pregnancies), your height and the shape of your pelvis.

These coming weeks are now crucial to your baby’s growth as he is now fully developed and is just a smaller, thinner version of what he is going to be when he is born , although he is not able to survive external of the womb yet as it is essential for his lungs to mature further. Your baby is also still looking a little red and wrinkled in pregnancy week 21 but is now developing extra layers of skin.

One of the discernible differences now is their high level of activity and the strength in their hands as he can be seen to be pulling, tugging and squeezing on his first toy…his umbilical cord. There is no danger of him bringing harm to himself though as the cord is thick and very rubbery.

Your 21 week fetus will now have limbs that are fully developed, there are 2 possible ways of measuring him; either ‘crown to rump’ (head to bottom) or ‘crown to heel’ (head to toe). Staying consistent with the measurements though, your baby will measure roughly 6.9” (17.5 cm) crown to rump by the end pregnancy week 21 and will weigh in at around 13.5 oz (383 g).

As for your size, the expansion of your belly may have been a little irregular up to this point but your uterus should be around the level of your navel by now. You should start seeing a regular increase in your size from now on.

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(See 21 Weeks Pregnant – The Mother’s Progress)

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