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 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.
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.
The fifth week of pregnancy is the time of the first missed period, when most women are only just beginning to think they may be pregnant. Yet already the baby’s nervous system is developing, and the foundations for its major organs are in place. At this stage embryo is around 2mm long.
As the ectoderm develops, a groove forms and the layer of cells folds to form a hollow tube called the neural tube. This will become the baby's brain and spinal cord. Defects in the "tail end" of the neural tube lead to spina bifida, while defects in the "head end" lead to anencephaly (when the bones of the skull do not 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.
By the time you are six to seven weeks pregnant, there is 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.
By seven 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 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 nervous system (the brain and spinal cord) starts to take shape.
By the time you're eight weeks pregnant, the baby is called a foetus, which means 'offspring'.
The legs are lengthening and forming cartilage too. 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 called chorionic villi that help attach the placenta to the wall of the womb. At this stage, the fetus still gets its nourishment from the yolk sac.
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.
Things to think about
Finding out you’re pregnant
The most reliable way of finding out whether you’re pregnant is to take a pregnancy test. Once you think you could be pregnant, it’s important to get in touch with a midwife or doctor to start your antenatal (pregnancy) care.
Help and advice for teenagers
Discovering you’re pregnant can be tough, but there is help out there.
Common pregnancy problems
From morning sickness to vaginal bleeding, find out how to cope with the minor and more serious symptoms that can occur in pregnancy.
Your feelings and relationships
Pregnancy is a time of physical and emotional changes that can affect your relationships, so get as much information and advice as you can to help you cope.
The best way to make sure both you and your baby stay healthy is to make sure you get all the care available to you during pregnancy. This includes scans and checks, screening, and free dental care.
You can save a to-do list online to keep track of things to do, such as taking folic acid and getting free dental care.
Pregnancy week by week
Find out what's happening to you and your baby at:
9, 10, 11, 12 weeks pregnant
13, 14, 15, 16 weeks pregnant
17, 18, 19, 20 weeks pregnant
21, 22, 23, 24 weeks pregnant
25, 26, 27, 28 weeks pregnant
29, 30, 31, 32 weeks pregnant
33, 34, 35, 36 weeks pregnant
37, 38, 39, 40 weeks pregnant
Over 40 weeks pregnant