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, the 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 wall of the womb, establishing a rich blood supply. This ensures the baby receives all the oxygen and nutrients it needs.
Conception usually takes place about two weeks after your last period, around the time that you ovulate (release an egg). In the first four weeks of pregnancy you probably won’t notice any symptoms. The first thing most women notice is that their period doesn't arrive. Find out about the signs and symptoms of pregnancy.
By the time you are eight weeks pregnant, you will probably have missed your second period. However, some women experience a little bleeding during the early weeks of pregnancy. Always mention any bleeding in pregnancy to your midwife or GP, particularly if it continues and you get stomach pain.
Your womb has grown to the size of a lemon by the time you are around seven or eight weeks pregnant. You're probably feeling tired. Your breasts might feel sore and enlarged, and you are probably needing to pass urine more often than usual.
Some pregnant women start to feel sick or tired, or have other minor physical problems for a few weeks around this time. Most women stop having morning sickness and start to feel better by the time they are around 14 weeks pregnant.