The first 12 weeks of pregnancy is called the first trimester.
Weeks 1 to 2 - Preparing for ovulation
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. This will depend on the length of your menstrual cycle.
Week 3 - Fertilisation
Once you release an egg, it begins to travel down the fallopian tube. This is the tube that transports the egg from the ovary to the womb. After sex, there may be sperm in the fallopian tube. At the moment of conception, one of the sperm enters and fertilises the egg.
After fertilisation, the fertilised egg continues to move towards the womb. It begins as a single cell that divides again and again.
By the time the fertilised egg reaches the womb, it has become a hollow ball of cells known as a blastocyst. Once the blastocyst reaches the womb, it will develop into an embryo. This is called implantation.
Week 4 - Implantation
In weeks 4 to 5 of early pregnancy, the blastocyst grows and develops within the lining of the womb. The outer cells reach out to form links with the mother's blood supply. After some time, they will form the placenta (afterbirth). The inner group of cells will develop into the embryo. These inner cells form three layers at first.
Each of these layers will grow to be different parts of the body:
- inner layer - this becomes the breathing and digestive systems and includes the lungs, stomach, gut and bladder
- middle layer - this becomes the heart, blood vessels, muscles and bones
- outer layer - this becomes the brain and nervous system, the eye lenses, tooth enamel, skin and nails
In these early weeks, the embryo attaches to a tiny yolk sac. This sac provides nourishment to the embryo. A few weeks later, the placenta will form in full and will take over the transfer of nutrients to the embryo.
Cells from the placenta grow deep into the wall of the womb. Here, they establish a rich blood supply. This makes sure the embryo receives all the oxygen and nutrients it needs.
Week 5 of pregnancy
This is the time of the first missed period. This is when most women are only beginning to think they may be pregnant.
The embryo produces more of the pregnancy hormone (hCG) - this causes your ovaries to stop releasing eggs. Your ovaries will also produce more oestrogen and progesterone - these hormones stop your period and help the placenta (afterbirth) to grow.
The pregnancy hormone, hCG, is present in your urine (wee) too. You may have enough of the hormone in your urine at this stage to turn a home pregnancy test positive.
At this stage, the nervous system is already developing. The foundations for the major organs are also in place. The embryo is around 2mm long and is about the size of a sesame seed.
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 brain and spinal cord.
At the same time, the heart is forming as a simple tube-like structure. The embryo already has some of its own blood vessels and blood begins to circulate. A string of these blood vessels connects you to the embryo, and will become the umbilical cord.
Week 6 of pregnancy
At 6 to 7 weeks, the embryo develops 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.
You can sometimes see the heart 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. The embryo has a thin layer of see-through skin. By the end of week 6, the embryo is about the size of a lentil.
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. The brain and spinal cord (the nervous system) starts to take shape. By the end of week 7, the embryo is about the same size as a pea.
Week 8 of pregnancy
By the time you're 8 weeks pregnant, the embryo is called a 'fetus'.
At this stage, the legs are getting longer and look a little like paddles. The different parts of the leg aren't distinct yet. It'll be a bit longer before the knees, ankles, thighs and toes develop.
The fetus is still inside its amniotic sac. The placenta continues to develop and forms structures that help attach the placenta to the wall of the womb.
The fetus is still getting its nourishment from the yolk sac. By the end of week 8, the embryo is roughly the same length as a raspberry.