From conception to birth
Ovulation happens when an egg is released from one of the ovaries. At the time of ovulation the mucus in the cervix is thinner than usual to let the sperm pass through more easily.
Fertilisation takes place in the fallopian tube when a single sperm and egg meet and fuse together to form a single cell called a zygote.
Within 24 to 36 hours of being fertilised the single cell divides into two cells, then 12 hours later into four cells, and so on.
The zygote continues dividing to form a cluster of cells called a morula. At around 3 to 4 days after fertilisation the morula leaves the fallopian tube and enters the uterus.
3 weeks pregnant
About six days after fertilisation the cluster of cells forms a hollow cavity, known as a blastocyst. The blastocyst burrows itself into the uterus lining. This process is called implantation. The woman is now considered to be three weeks pregnant because it is approximately three weeks since her last period.
4 weeks pregnant
Size: about 4mm
The inner group of cells is now called an embryo. The outer cells reach out like roots to link with the mother’s blood supply to form the placenta. The inner cells form into two and then later into three layers. Each of these layers will grow to be different parts of the body.
5 weeks pregnant
Size: about 7mm
The cells fold up and round to make a hollow tube called the neural tube.
This will become the baby’s brain and spinal cord. By the end of this week, blood circulation will begin and the heart will develop quickly. The woman ‘misses’ her period.
6–7 weeks pregnant
Size: about 8-10mm
The brain is developing distinct areas, and the eyes and ears are beginning to take shape. The heart begins to beat and can be seen beating on an ultrasound scan. Small swellings called ‘limb buds’ show where the arms and legs are growing.
8–9 weeks pregnant
Size: about 22mm
The baby is now called a foetus, meaning ‘young one’. The face is slowly forming. The eyes are now more obvious and have some colour in them. There are now the beginnings of hands and feet, with ridges where the fingers and toes will be. The major internal organs are all developing.
10–12 weeks pregnant
Size: about 85mm
Just 12 weeks after conception the foetus is fully formed. By now, almost all of the baby’s organs and structures have formed and will continue to grow until delivery. The baby is already moving about, but the movements cannot yet be felt.
13–20 weeks pregnant
Size: about 15cm
The baby is now growing quickly. The body grows bigger so that the head and body are more in proportion. The face begins to look much more human and the hair is beginning to grow as well as eyebrows and eyelashes. The eyelids stay closed over the eyes.
21–24 weeks pregnant
Size: about 27cm
At about 22 weeks the baby becomes covered in a very fine, soft hair called lanugo. The purpose of this is not known, but it is thought that it may be to keep the baby at the right temperature. It may be possible to feel the baby move for the first time.
25–26 weeks pregnant
Size: about 29cm
The baby is now moving about vigorously and responds to touch and sound. The baby may also begin to follow a pattern for waking and sleeping. At around 26 weeks the baby’s eyelids open for the first time.
27–29 weeks pregnant
Size: about 33cm
The baby’s heartbeat can now be heard through a stethoscope. The baby is now covered in a white greasy substance called vernix. It is thought that this may be to protect the baby’s skin as it floats in the amniotic sac.
30–31 weeks pregnant
The baby is growing plumper, so the skin which was quite wrinkled before, is now smoother. Both the vernix and the lanugo begin to disappear.
32 weeks pregnant
The baby is now beginning to prepare for birth. He or she will often turn downwards ready for the birth at around this time.
33–42 weeks pregnant
Some time before the birth, the head may move down into the pelvis and is said to be ‘engaged’. The average size of a full-term baby is 2.7 to 4.1kgs and 50-53cms long at birth. The placenta is about one-sixth of the baby’s weight, and the umbilical cord is almost as long as the baby.
NHS Choices 2011