First Trimester 2^nd Trimester     3^rd Trimester

Fertilization

Egg

The sperm's head carries genetic material along with enzymes that enable the sperm to penetrate the egg's tough outer layer.

The whiplike tail of the sperm propels it up the fallopian tube to meet the egg.

Cells surrounding the egg secrete chemicals that aid egg development.

Start of Implantation

Fluid-filled cavity

Blastocyst

Lining of uterus

Cell cluster within the blastocyst's fluid-filled cavity forms the beginnings of the embryo.

The outer cells of the blastocyst anchor into the lining of the uterus.

Maternal blood vessels

If you ovulated and the egg met a sperm, amazing things will happen fast. It takes just three days from fertilization for a single egg to divide into a ball of 58 cells. By the end of the week, this ball, called the blastocyst, will have reached the uterus, where it will start to implant in the lining. It will be a couple of weeks before you know whether you've conceived, but special hormones kick in now to help maintain the pregnancy.

Excerpted from Pregnancy Day by Day.
Copyright © 2009 Dorling Kindersley Limited.
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