Weeks 0 – 3
Your weeks of pregnancy are dated from the first day of your last period. This means that in the first two weeks or so, you aren’t actually pregnant — your body will be preparing for ovulation as usual. You ovulate (release an egg) around two weeks before the first day of your next period (depending on the length of your menstrual cycle).
During the third week after the first day of your last period, your fertilized egg moves along the fallopian tube towards the womb. The egg begins as a single cell, which divides again and again. By the time the egg reaches the womb, it has become a mass of more than 100 cells, called an embryo. Once in the womb, the embryo burrows into the lining of the womb. This is called implantation. The cells that will become the placenta start to develop the pregnancy hormone hCG (human chorionic gonadotropin), which gives a positive reading in a pregnancy test. Amniotic fluid is starting to form.
In weeks four to five of early pregnancy, the embryo grows and develops within the lining of the womb. The outer cells reach out to form links with the mother’s blood supply. The inner cells form into two, and then later, into three layers. Each of these layers will grow to be different parts of the baby’s body.
The inner layer, called the endoderm, becomes the breathing and digestive systems, including the lungs, stomach, gut, and bladder. The middle layer, called the mesoderm, becomes the heart, blood vessels, muscles, and bones. The outer layer, called the ectoderm, becomes the brain and nervous system, eye lenses, tooth enamel, skin, and nails.
In these early weeks of pregnancy, the embryo is attached to a tiny yolk sac which provides nourishment. A few weeks later, the placenta will be fully formed and will take over the transfer of nutrients to the embryo.
The embryo is surrounded by fluid inside the amniotic sac. It’s the outer layer of this sac that develops into the placenta. Cells from the placenta grow deep into the lining of the womb, establishing a rich blood supply. This ensures the baby receives all the oxygen and nutrients it needs.