Foetal development

In the very early weeks, the developing baby is called an embryo. Then, from about eight weeks onward, it is called a foetus, Pregnancy is broken down into weeks, and full term pregnancy is considered 37 to 40 weeks and an infant delivered before the 37th weeks is considered premature. Pregnancy is counted from the first day of your period. This means that at conception, around the second week, you are already considered two weeks pregnant

Week 1

This is the week of your last menstrual period. The first day that you bleed is considered the start date of your 40 week pregnancy, until the dating scan later in the pregnancy.

Week 2 - Fertilisation

Every month, about three to 10 eggs receive a hormonal signal that cues them to mature and prepare for ovulation. At the same time, the uterus is forming a blood rich lining that is called the endometrium. This will thicken, upon fertilization, to form a sack strong enough to carry and cushion your baby throughout the entire pregnancy. In one of the ovaries, the eggs ripen in fluid filled sacs called follicles. Around the 14th day, one of the eggs is released and travels down into the fallopian tube. If in the next 36 to 48 hours, just one of the 350 million sperm in the average ejaculate can get all the way from the vagina through the uterus and into the fallopian tube to penetrate the egg, the egg will become fertilized.

Week 3

The fertilised egg moves slowly along the fallopian tube towards the womb. The egg begins as one single cell. This cell divides repeatedly and by the time the egg reaches the womb it has become a mass of over 100 cells, called an embryo, and is still growing. Once in the womb, the embryo burrows into the womb lining. This is known as implantation.

Week 4

At week 4 the embryo is 5mm in size. The embryo now settles into the womb lining. The outer cells reach out like roots to link 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. One layer becomes the brain and nervous system, the skin, eyes and ears. Another layer becomes the lungs, stomach and gut. The third layer becomes the heart, blood, muscles and bones.

Week 5

The fifth week 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 starting to develop. This is when the placenta and umbilical cord hook up to the wall of the womb. Your baby's heart is about the size of a poppy seed and it starts beating.

Week 6

By week 7 the embryo is 10mm in size. There is now a large bulge where the heart is and a bump for the head because the brain is developing. Dimples on the side of the head will become the ears and there are thickenings where the eyes will be. On the body, bumps are forming which will become muscles and bones. And small swellings show where the arms and legs are growing.

Week 7

At seven weeks the embryo has grown to about 10 mm long, about the size of a grain of rice. Development of the arms and legs continue although the fingers and toes haven't yet formed. The brain is growing as well as the lenses of the eyes, nostrils, intestines, pancreas.

Weeks 8–9

A face is slowly forming. The eyes are more obvious and have some

colour in them. There is a mouth and a tiny tongue begins to form. There are now the beginnings of hands and feet, elbows begin to form in the arms and fingers start to develop. The leg buds begin to show feet with tiny notches for the toes. The major internal organs are all developing the heart, brain, lungs, kidneys, liver and gut. At nine weeks, the baby has grown to about 22 mm long from head to bottom, and weighs 2g.

Week 10

At ten weeks the foetus is around 31mm and weighs 4g. The baby has begun moving inside the womb although it's too small for mum to notice. Most of the joints are formed now, elbow, wrist, knee, shoulder, and ankle as well as the hands, fingers, feet and toes.

Week 11

At 11 weeks the foetus is roughly the size of a thumb or 41mm and weighs 7g, it can swallow and kick, and its fingernails have started to grow. Its head is so big it takes up almost half the length of its body, and a clear outline of its spine is visible.

Week 12

At this point the foetus is around 54mm and weighs about 14g. Just 12 weeks after conception the foetus is fully formed. It has all its organs, muscles, limbs and bones, and its sex organs are well developed. From now on it has to grow and mature. The baby is already moving about, but the movements cannot yet be felt. By the end of this week, your uterus is too large to remain completely in your pelvis.

Week 13 -16

Your baby's head is still the largest part of their body. Blood starts to form in their bone marrow, eyes become well formed and genitalia are now distinctive. By the 16th week of gestation, soft fine hair known as lanugo starts to cover their skin, they measures around 11cms and weighs around 100g. The heartbeat is strong and can be heard by using an ultrasound detector. The heartbeat is very fast about twice as fast as a normal adult’s heartbeat. The lines on the skin of the fingers are now formed, so the baby already has its own individual fingerprints. Fingernails and toenails are growing and the baby has a firm hand grip.

Weeks 17 - 20

Your baby's body now grows more quickly than their head, bones are more distinctive and muscles become stronger. The increase of blood formation continues in the bone marrow. The testes in a boy and womb and vagina in a girl are now established. By the 20th week of gestation, your baby measures around 16cms, weighs around 300g. You may even feel the baby hiccoughing

Week 21 - 24

Your baby's internal organs are now organised into their lifelong positions. The large intestine and cerebral cortex are recognisable and the nostrils have opened up. Your baby will now have noticeable times of sleep alternating with activity and may respond to sounds. This is the time when babies are often seen to begin sucking their thumbs! By week 24 your baby measures around 30 cms and weighs around 600g.

Weeks 25 - 28

The baby is now moving about vigorously and responds to touch and to sound. A very loud noise close by may make it jump and kick. It is also swallowing small amounts of the amniotic fluid in which it is floating and passing tiny amounts of urine back into the fluid. The baby’s heartbeat can now be heard through a stethoscope. Your partner may even be able to hear it by putting an ear to the belly, but it can be difficult to find the right place. 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 fluid. The vernix mostly disappears before the birth. At 24 weeks, the baby is called ‘viable’. This means that the baby is now thought to have a chance of survival if born. At around 26 weeks the baby’s eyelids open for the first time. The head to bottom length at 30 weeks is about 40cm and around 1.3kg.

Weeks 29 - 32

Your baby is getting larger it is growing fat layers in preparation for birth, their body is becoming rounder and the skin a lot less wrinkled. You will feel plenty of movement during this time as the baby still has room to manoeuvre around in your womb. By the 32nd week of gestation, your baby will measure around 42cms and weighs 1.7kg. Both the vernix and the lanugo begin to disappear. By about 32 weeks the baby is usually lying downwards ready for birth.

Weeks 33 - 36

The baby’s brain and nervous system are now fully developed. Your baby's bones are also starting to harden, even though the skull bones will stay soft and separated to make the journey through the birth canal easier. Baby boys’ testicles are now beginning to descend from the abdomen into the scrotum. By 36 weeks the baby is around 47cm and weighs 2.6kg

Weeks 37 - 42

Babies are considered full term when they are born 37+ weeks. This means that a baby born 37 weeks and on can survive in the outside world even though the due date may still be three weeks away. Many babies have a full head of hair at birth, but don't be surprised if your baby's hair isn't the same colour as yours or your husbands or if it is sporting light fuzz their true hair colour will not show for a while and remember most blonde babies darken as they get older. The baby size at birth will depend on when it is born and ranges somewhere from 6 pounds, 2 ounces (2.8kg) to 9 pounds, 2 ounces (4.1kg). Their average length ranges from 19 to 21 inches (48 to 53 cms).