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Pregnancy health centre

3 weeks pregnant

WebMD Medical Reference
Medically Reviewed by Dr Rob Hicks

egg being fertilized

How is your baby growing at 3 weeks?

If the egg was not fertilised at the end of week 2, it should be before the middle of week 3. After sexual intercourse, sperm travel through the uterus and up to the fallopian tube - the tube that connects the ovary to the womb. The moment of fertilisation occurs when a single sperm penetrates inside the egg, forming a single cell known as a zygote. At this stage the sex of the baby will already be determined, and chromosomes from both the egg and sperm will be mapping out how your baby develops – including the baby's eye and hair colour.

The zygote will divide into two cells 24–36 hours later, just a mere 0.1–0.2mm in total, then into four cells after about another 12 hours. This dividing of cells will continue, forming a cluster of 16 cells known as a morula (meaning "mulberry shaped").

The morula will enter the uterus 2 to 3 days after the egg is fertilised, with cell division a continuing process. At this stage it will be known as a blastocyst – it will be hollow, filled with a fluid and have more than 100 cells. There will be two types of cells: the inner mass of cells is the embryoblast and the outer layer of cells is the trophoblast.

The blastocyst will attach itself to the lining of the wall of the uterus about 6 days after fertilisation – this is the big moment of implantation, when the developing baby attaches itself to the mother's womb. The outer layer of trophoblast cells will be responsible for forming the placenta, which only becomes fully functional several weeks later.

The timing of implantation depends on when ovulation and fertilisation occurs, but it generally occurs towards the end of week 3 from the start of your last period, though in women who ovulate later in their cycle implantation can occur in the beginning of week 4.

How are you changing in week 3?

Some women experience light spotting at the time of implantation – this is sometimes mistaken for an early, light period. Otherwise you're unlikely to notice anything being different and probably won't know that you are pregnant.

However, important changes have already begun inside your womb. The trophoblast cells of the blastocyst will stimulate the corpus luteum (the collapsed follicle) to continue making the hormone progesterone, which will maintain the blood-rich lining of the womb. New blood capillaries in the uterus will make it swell and allow circulation to form between mum and the blastocyst.

What you need to know in week 3

Though you may not know if you're pregnant, if you are planning for a baby try to make sure you are eating a healthy diet that includes protein and calcium, and are taking a daily folic acid supplement. Protein is used to form new tissues and calcium for developing bones and teeth.

Next: 4 weeks pregnant

Additional pregnancy week by week pages:
5 weeks pregnant 6 weeks pregnant 7 weeks pregnant 8 weeks pregnant

Reviewed on January 16, 2017

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