Week Five: The embryonic stage begins

You are 5 weeks pregnant. (about three weeks from conception)

• The amniotic fluid is being built up.
• The embryo grows from a tiny speck about 2mm - the size of a pin head.
• By the end of the week the embryo will have more than doubled in size to about 4-5mm.
• The egg has now completely grown into the mucous membrane of the uterus which covers the egg, the umbilical sac and the amniotic fluid cavity.
• A fibrin clot closes the hole to the uterus cavity.
• The placenta is developing.
• The embryo starts receiving oxygen and nutrition through the placenta.

The cells making up the ovum are beginning to differentiate and specialize in areas of development. Some cells will make up the embryo; others the amniotic sac and placenta etc. The ovum, which has been floating around in your uterus, has now implanted into the wall of the uterus. Now comes an exciting time of rapid growth! The amniotic sac, amniotic cavity and yolk sac are developing. The placenta is beginning to form now too.

Implantation - Some spotting (also known as implantation bleeding) may occur about 10 - 14 days after conception. You may believe you are starting your period but generally this spotting (bleeding) is extremely light and lasts only a day or so. At week five, you might begin to suspect you're pregnant since the embryo produces hormones which stop the mother's menstrual cycle. The fertilized egg, now called a blastocyst, is a fluid-filled cluster of about 500 cells, still multiplying madly. It is attached to the uterus wall and divides into two parts. The half attached to the uterine wall becomes the placenta, the vessel-filled support system that nourishes the developing life, and the other half will become the baby.

At the end of week five; nerve growth begins when a sheet of cells on the back of the embryo folds in the middle to form a tube, which will become the future spinal cord. At one end tube enlarges to form the brain's major sections. The amniotic fluid that cushions the embryo begins to form. Between the forth and fifth week the embryo more than doubles in size to about, 4-5mm about a 1/8 inch in size.