The first two weeks of pregnancy - online guide

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Pregnancy calendars give you an idea of how your body is changing and your baby developing week by week during your pregnancy. This page looks at the first two weeks following fertilization while our page on the first trimester of pregnancy brings you up to week 12, and the pregnancy week by week page looks at changes from week 13-40.

The first 20 hours

Soon after the egg is fertilized, the cell that has been formed begins to divide in two. The division continues rapidly, and each time the cell divides the DNA is replicated. The fertilized egg is called a zygote and the bundle of cells that form it are about the size of a pinhead. If the zygote splits at this stage it will form identical twins. Twins occur naturally in about one in 80 births.

The first few days

A pregnancy calendar will highlight the enormous changes that are going on in the first few days after conception. As the bundle of cells divides and replicates it moves towards the uterus where the fetus will eventually grow. The cells (now called a morula) reach the uterus about 90 hours after conception. By this stage the morula will contain about 64 cells. Only a few of these cells go on to form the embryo, the others will form the placenta and membranes than surround the baby in the uterus. The morula then changes from a solid form into a fluid-filled ball called a blastocyst. The cells that will become the embryo are at the centre of the ball.

Five to nine days after conception

At this point progesterone levels are at their highest, triggering the growth of rich blood vessels that supply the lining of the uterus. The blastocyst floats freely in the uterus for a few days and continues to grow to several hundred cells. Approximately nine days after fertilization the blastocyst attaches itself to the uterus wall. This can sometimes cause some minor bleeding known as spotting. If the blastocyst does not attach it will be washed out with the next menstrual period. About 40 per cent of blastocysts fail to implant.

After implantation the placenta begins to develop and provide nourishment to the cells. The placenta begins to produce the hormone human chorionic gonadotrophin (HCG) that can be detected by pregnancy tests. A pregnancy calendar will help you determine when you can take a test and expect a true result.

Ten to 14 days after conception

By about day 13 the embryo will be firmly implanted in the lining of the uterus. The embryo begins to produce its own progesterone triggering further growth of the uterus lining. Organs such as the nervous system and the heart soon begin to develop. Thirteen days is the latest date that an embryo can split to become twins, if it splits later than this it will form conjoined (Siamese) twins.

© The Pink Fridge Online Magazine 2008 - The First Two Weeks of Pregnancy