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At 4 weeks
The embryo has a head and a tail end, a back and a front. Tiny swellings indicate the beginnings of arms and legs. The heart begins to beat. The embryo is 5 mm (0.2 in.) long.
At 8 weeks The major organs of the body are formed, the limbs, hands and feet have formed. The head is large and facial features are clear. The embryo will have started to grow all it needs to be a human baby. It will be 3 or 4 cm (1-1.5 in.) long and weigh about 10 g (0.35 oz).
Between 8 and 12 weeks
The embryo is now called a fetus. During this stage it is developing rapidly. It has a heart, a brain, a digestive system, bones and muscles, and the brain can now give instructions to the bones and organs enabling the fetus to kick, swallow, and move its arms and head. The tooth buds appear. The kidneys function, allowing some urine to be passed into the amniotic fluid. The distance from the top of the fetus’ head to the base of the spine (crown-rump length) is about 71/2 cm (3 in.) and it weighs about 28 g (1 oz). It is now possible to tell the sex of the fetus.
At 16 weeks
At four months the fetus is still floating freely in the amniotic sac. The growth of the skull is rapid, and an ultrasound examination at this stage can detect abnormalities such as spina bifida. The expectant mother can be offered an abortion if an ultrasound-scan reveals that the fetus is abnormal. The fetus’ skin is getting thicker and the bones are hardening. It weighs about 135 g (4.8 oz).
Between 16 and 24 weeks
Very fine hair (lanugo) begins to grow over the skin of the fetus, and the eyelashes and hair on the head grow. The eyelids are still closed. Fat under the skin makes the fetus less wrinkled, and it develops a white greasy covering (vernix). The fetus’ movements are becoming more energetic and the expectant mother will be able to feel them. It will be lying in the foetal position, with back bent and knees drawn up towards the chin. Up to this stage the fetus is pre-viable – that is, it would stand no chance of survival if born, because the vital organs are incapable of working properly. The fetus weighs about 400-500 g (14-17.5 oz) at 20 weeks and about 600 g (21 oz) at 24 weeks.
By the end of this stage the fetus’ heartbeat can be heard through a special stethoscope.
At 28 weeks
The fetus is about 24 cm (9.4 in.) long, weighs about 800-1000 g (1 lb 12 oz-2 lb 3 oz), and is considered viable. From this time, a fetus’ death must be registered. It has been recommended that this should be brought down to 24 weeks, as some foetuses are viable at this stage. In a good special care baby unit a 28-week fetus has a 70 per cent chance of survival.
This is the end of the second trimester, or gestation period.
Up to 32 weeks
The fetus has a very good chance of survival if born prematurely at this stage. It will be able to withstand almost all outside influences including X-ray, ultrasound (both may be dangerous in the early months of pregnancy), shock, knocks and drugs, but it is suspected that the expectant mother’s smoking can harm the still-developing cells of the fetus’ brain. Any infection will have less severe effects than at earlier stages. The head and body are in the same proportions as in a newborn baby and the lungs are maturing, although the fetus is not yet breathing. It weighs about 1600 g (3 lb 8 oz), and is getting too big to float about freely.
At 36 weeks
All the vital organs have matured, a lot of fat has been laid down and early wrinkling of the skin has disappeared. The lanugo has nearly disappeared and the fetus is making normal physical movements (e.g., clutching its fingers, kicking, etc.). It should have moved into the correct position for birth, with the head down towards the cervix so that the crown of the baby’s head (vertex) will appear in the vagina first. The fetus now weighs about 2.5 kg (5 lb 8 oz) and the crown-rump length is about 31 cm (12 in.).
The 9th month (Up to 40 weeks)
During the last few weeks of pregnancy the hair on the fetus’ head is thicker and the bones of the skull become harder, although they remain soft enough to be squeezed through the cervix at birth. By the 40th week the fetus is fully developed. It may still have a slippery greasy covering but this may disappear if the fetus is in the uterus for over 40 weeks. The head will move lower down the uterus with the spine parallel to the mother’s. In two to three per cent of pregnancies the presentation (position) of the fetus is with the bottom down towards the cervix, which can cause complications at birth. This is known as the breech position.
A full-term fetus (40 weeks) weighs from 2.8 kg-4 kg (6 lb 2 oz-8 lb 12 oz). Although it is not fully understood what triggers off the birth, it is thought that this moment occurs when the fetus is mature enough to survive the outside world.
How to Keep Track of Your Pregnancy Week - By - Week
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