3 weeks pregnant: Changes for baby

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The single 46 chromosome cell from which your baby grows is at this stage known as a zygote. It forms when the sperm and egg fuse and will go on to produce more than 2 trillion new baby cells by the time you give birth to your bundle of joy. It does this by dividing and replicating. The single cell will divide and replicate its chromosomes to form 2 new cells within about 12 hours of conception. Thereafter, the number of cells will double approximately every twelve hours.

How big is your baby now?

The single cell begins to divide and replicate and by three days after conception (day 18 of your pregnancy if fertilisation occurred on day 15) there are 13-32 cells, shaped in a ball which looks something like a tiny raspberry. This structure is called the morula. It will increase to a size of approximately 500 cells by day 4 or 5 following fertilisation. At this stage it remains less than a millimetre in length. To put this in perspective, this is the thickness of about 8 sheets of paper.

What is your baby doing?

The morula moves through the fallopian tubes to the uterus (womb) on day 18 (three days after fertilisation) at which stage it becomes known as a blastocyst. The blastocyst consists of a sack of liquid containing two groups of cells, one group which will develop into your baby, and another which will grow into the placenta, that is the organ which nourishes your baby as it grows inside you.

On days 19-20 (4-5 days after fertilisation) the fertilised egg sheds its shell. Although a sperm penetrated the egg shell when fertilisation occurred on about day 15, at that stage the shell remained in place to protect the egg and sperm. The shell is only shed on day 19-20 as the fertilised egg prepared for implantation in your womb. The lining of your womb has been thickening in weeks 1-2 of pregnancy and is now full of the nutrients which will feed the beginnings of your new baby (now called the blastocyst) from now until the placenta forms in week 5.

Once inside your womb, the fertilised egg begins releasing enzymes which eat away at the lining of the uterus, creating a space in which the blastocyst can implant itself. When your baby begins implanting in your uterus, it may cause some spot bleeding or a yellow vaginal discharge. This occurs when small amounts of the lining of the uterus break away to make space for the blastocyst to implant. You may mistake this bleeding as the beginning of your period, although it will most likely occur before you menstrual bleeding is due.

The blastocyst (which will grow into the embryo) begins to implant in the uterus at the end of week 3, but at this stage remains only superficially attached. Because the attachment is superficial, it is easy for the blastocyst to become unstuck from the wall of the uterus, which causes miscarriage. Miscarriage is very common in this very early period of pregnancy and scientists believe that 3 of every 4 pregnancies result in miscarriage at this time when the woman not yet aware she is pregnant, because the blastocyst fails to implant properly into the lining of the uterus. However, at this very early stage a miscarriage appears the same and at the same time as normal menstrual bleeding.


More information on the 3rd week of pregnancy

For more information on the third week of pregnancy, see 3 weeks pregnant.
For more information about the changes that occur when pregnant, see Changes to mum this week.
For more information about preparing your body for pregnancy with healthy eating and lifestyle changes, see Pregnancy health information this week.
For more information about the key points on the third week of pregnancy, see Key points this week.
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Date Created: August 12, 2012 Date Modified: December 20, 2012