Embryo of approx. 8 weeks gestational age
Image by lunar caustic, cropped (CC BY 2.0)

Once a month, a microscopically small egg (ova) is released from one of a woman’s ovaries and travels down the Fallopian tube towards the uterus. If during this journey the egg encounters sperm released by the woman’s partner, the egg may be fertilised, and the woman becomes pregnant. Once penetrated by the sperm, the egg starts multiplying, from one cell to two, then four, eight, 16, and so on, doubling in size with each division.

Initially the fertilised cell mass is called a zygote. As the cells continue to multiply the ball of cells is called a morula, and then as a hollow develops in the centre of the ball, a blastocyst. After ten days, the growing embryo consists of a fluid-filled ball, only a couple of millimetres across. At this point it implants into the endometrium lining the inside of the uterus (a process called nidation) and continues to grow, drawing all it needs from the mother through the placenta.

For the first 12 weeks, the developing baby is called an embryo. The growth of the embryo is rapid to start with, but slows down as maturity approaches. The embryo soon becomes the size of a grain of rice, and then a tadpole (both in size and appearance). By the end of the first month, it is about eight millimetres long, with four small swellings at the sides, called limb buds, which will develop into arms and legs.

At eight weeks of pregnancy, the embryo is 2 cm long, and the nose, ears, fingers and toes are identifiable. Most of the internal organs form in the next four weeks, and by 12 weeks when the baby is 5.5 centimetres long, a pumping heart can be detected, and the baby is moving, although too weakly yet to be detected by the mother. It is during the first three months that the embryo is most prone to the development of abnormalities caused by drugs (eg. thalidomide, isotretinoin) or infections (eg. german measles).

Once it is three months old the baby is called a foetus.

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(Last modified: 10th Oct 2014)

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