How the baby develops during pregnancy

Fertilisation

You can get pregnant if you have intercourse with a man around the time you ovulate – from about five days before until one day after. When the man ejaculates inside your vagina, the sperm swim up the vagina through the cervix – the neck of the womb – through the womb, or uterus, to the fallopian tubes.

It takes the sperm about 10 hours to make this journey. If there’s an egg waiting in one of the fallopian tubes, the tiny sperm try to burrow their way inside it. If one sperm gets inside the egg, it’s fertilised. The fertilised egg then sets off down the fallopian tube to the womb.

Week 1

The fertilised egg cell starts dividing – at this stage it’s called a zygote. After a few days of division it becomes a ball of cells called a morula. You’d need very good eyes or a microscope to see it.

The blob of cells develops a hole in the middle full of fluid, at which point it gets another new name: blastocyst. First it floats around the walls of the womb for a while. At this stage it could still just get flushed out of your womb with your period, and you’d never know anything had happened. But then comes a crucial moment: if the womb accepts the blastocyst, and it nestles into the lining, you’re pregnant.

Even then, all sorts of things can go wrong in the early stages of a pregnancy. You might notice nothing more than your period coming a couple of days late, when in fact you were nearly going to have a baby. It’s only called a miscarriage if happens later on, after you know you’re pregnant.

Week 2

The pregnancy is now properly established. Inside the blastocyst, different groups of cells develop, which will eventually form the different parts of the baby. The inner cells will grow to become lungs, stomach and intestines, a middle layer will be muscles and bones, and an outer layer will form the nerves and skin.

Its time for the blastocyst to have another new name. It’s now an embryo. The end of the second week is about day 28 of your menstrual cycle – that means your period is due. And at this stage, depending on whether or not you wanted to get pregnant, you’re likely to start getting nervous and worried, or hopeful and excited. Because your period is late.

From a couple of days before your period was due, pregnancy hormone starts getting into your urine. So as soon as your period is late, you can do a pregnancy test.

Week 3

The bundle of cells starts developing the first signs of the different parts of the body – the brain and heart start to grow.

Week 4 to 8

The heart starts beating, and arms, legs, eyes and ears start to form. Nipples appear, and the kidneys start producing urine. The foetus starts moving. By the end of week eight, the embryo is about the size of a small grape – about 13mm. And once again it’s time for a name change: the embryo is now called a foetus.

Week 9 to 12

The foetus now develops a recognisable human face. Its arms and legs are properly formed and it can make its hands into a fist. What’s more, the foetus can even make sounds – even though it’s still no longer than your thumb.

Twelve weeks marks the end of what’s known as the first trimester, the first third of the pregnancy. Miscarriages are much more likely to happen in the first twelve weeks, so women sometimes prefer to wait to this point before telling everyone they know that they’re pregnant. In some countries where abortion is allowed, you can’t have the pregnancy terminated after this point, or only in special circumstances.

Week 13 to 19

The foetus starts moving around and making sucking movements with its mouth. By week 19 the baby is waking and sleeping, and can hear. It’s about 12 to 15cm long, so would still fit in the palm of your hand.

Week 20 to 24

Butterflies fluttering inside you, or bubbles – that’s how many women describe feeling the foetus moving inside them for the first time. It usually happens around this time of the pregnancy. However, with a first baby, you may not feel it until 25 weeks – and with a second baby it can be as early as 16 weeks.

Apart from swimming around, the foetus is now growing eyebrows and eyelashes, and nails on its fingers and toes. The senses of taste and touch are developing, and the eyes are properly developed too. With a lot of medical care, foetuses born at 24 weeks can sometimes survive.

Week 25 to 28

The brain is growing fast and by week 28 the lungs are properly developed, so if the baby is born at this stage it has a good chance of survival. Week 28 is the end of the second trimester, the middle third of the pregnancy.

Week 29 to 40

In this last stretch of the pregnancy, the third trimester, the foetus grows fast, and puts on weight ready for life outside the womb. It starts ‘breathing’ the amniotic fluid inside the womb.

The foetus hasn’t got much room to move around any more, but it can give some impressive kicks and you can see it moving from the outside. Around week 36, all being well, it swivels round so its head is pointing downwards ready for the birth.

The due date

People usually say a pregnancy lasts nine months. And based on the first day of your last period, you’re usually given a ‘due date’ when your baby is supposed to be born. But in fact most births happen anywhere between about 37 and 42 weeks. So within this period you can’t really say the birth is ‘early’ or ‘late’.

Anyway, at some point around 40 weeks the big moment arrives. Having been a zygote, a morula, a blastocyst, and an embryo, the foetus now has its last official name change of the pregnancy: it’s a baby. After that, the name is up to you!