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Weeks 1-3 of pregnancy
Your weeks of pregnancy1 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 after the first day of your period, depending on the length of your menstrual cycle.
During the third week after the first day of your last period, your fertilised 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.
Week 4 of pregnancy
In weeks 4-5 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 becomes the breathing and digestive systems, including the lungs, stomach, gut and bladder
- the middle layer becomes the heart, blood vessels, muscles and bones
- the outer layer 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 that 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.
Week 5 of pregnancy
Week 5 of pregnancy is the time of the first missed period, when most women are only just beginning to think they may be pregnant.
The baby's nervous system is already developing, and the foundations for its major organs are in place. At this stage, the embryo is around 2mm long.
The embryo's outer layer of cells develops a groove and folds to form a hollow tube called the neural tube. This will become the baby's brain and spinal cord.
Defects in one end (the "tail end") of the neural tube lead to spina bifida. Defects in the "head end" lead to anencephaly, when the bones of the skull don't form properly.
At the same time, the heart is forming as a simple tube-like structure. The baby already has some of its own blood vessels and blood begins to circulate. A string of these blood vessels connects the baby and mother, and will become the umbilical cord.
Week 6 of pregnancy
By the time you're 6-7 weeks pregnant, there's a large bulge where the heart is and a bump at the head end of the neural tube. This bump will become the brain and head. The embryo is curved and has a tail – it looks a bit like a small tadpole.
The heart can sometimes be seen beating on a vaginal ultrasound scan at this stage.
The developing arms and legs become visible as small swellings (limb buds). Little dimples on the side of the head will become the ears, and there are thickenings where the eyes will be. By now the embryo is covered with a thin layer of see-through skin.
Week 7 of pregnancy
By 7 weeks, the embryo has grown to about 10mm long from head to bottom. This measurement is called the crown-rump length.
The brain is growing rapidly and this results in the head growing faster than the rest of the body. The embryo has a large forehead, and the eyes and ears continue to develop.
The inner ear starts to develop, but the outer ear on the side of the head won't appear for a couple more weeks.
The limb buds start to form cartilage, which will develop into the bones of the legs and arms. The arm buds get longer and the ends flatten out – these will become the hands.
Nerve cells continue to multiply and develop as the brain and spinal cord (the nervous system) starts to take shape.
Week 8 of pregnancy
By the time you're 8 weeks pregnant, the baby is called a foetus, which means offspring.
The legs are getting longer. The different parts of the leg aren't properly distinct yet – it will be a bit longer before the knees, ankles, thighs and toes develop.
The foetus is still inside its amniotic sac and the placenta is continuing to develop, forming structures that help attach the placenta to the wall of the womb.
At this stage, the foetus still gets its nourishment from the yolk sac.
Your body in early pregnancy
Conception usually takes place about two weeks after your last period, around the time you release an egg (ovulate). In the first 4 weeks of pregnancy, you probably won't notice any symptoms.
The first thing most women notice is that their period doesn't arrive.
By the time you're 8 weeks pregnant, you'll 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're around 7 or 8 weeks pregnant. You're probably feeling tired. Your breasts might feel sore and enlarged, and you're probably needing to pee 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. For most women, feelings of nausea (morning sickness) start to improve by the time they're around 14 weeks pregnant.
1Contains public sector information licensed under the Open Government Licence v3.0.