Development before birth

A human egg is just large enough to be seen - about the size of full stop. After being fertilized, the egg soon starts to divide, first into two cells, than into four, then eight and so on, until it is a mass of cells. By this time it has become attached to the wall of the uterus. The number of cells continues to increase and gradually a tiny embryo forms.
Besides producing the embryo, the fertilized egg also gives rise to the placenta, umbilical cord and amnion. These structures are developed for the support of the baby, and they leave the uterus at birth.

The timing of pregnancy : Pregnancy is timed from the first day of the last menstrual cycle, not from conception.
So what is called 'week 6' or 'six weeks pregnant' is actually about four weeks after conception. Pregnancy normally lasts for 32-42 weeks from the first day of the last menstrual cycle, the average being 40 weeks.

Growth and development of the baby:

Week 6: At this stage a human embryo looks rather like the embryo of a fish or frog (tadpole). It is possible to see a tail, and parts with look as though they might develop into gills. The actual size of the embryo at this stage is like a peas.
Week 9: The embryo has grown to look more human-like and it is now called a fetus (or foetus). The main organs of the body are developing and the heart can be seen beating on an ultrasound scan.
Week 14: During weeks 10-14 the nerves and the muscles develop rapidly and by the end of this time, the fetus can swallow, frown, clench the first, and move by turning the head and kicking. The mother does not feel the movements at this stage. The fetus is now about the size of a mouse and weights about 55 g (2 oz.)
Week 20: about this time the mother is able to feel the movements inside the uterus as the baby practices using its muscles. The heartbeat can be heard, and very fine hair, (lanugo) covers the skin. The fetus weights about 350 g. (3/4 lb)
Week 28: Development is almost complete. The baby will spend the remaining time in the uterus growing larger and stronger. It will become more plump as a layer of fat is stored under the skin. The length will be doubled and the weight increased three times. By about week 32 the baby is usually lying head downwards and ready for birth. The fetal position is : the back curved. head forwards, knees bent, and arms crossed over the chest.

Inside the uterus: 
In the uterus, the baby develops in a 'bag of water' (amniotic fluid) which remains at a constant temperature of about 37 °C.

The drawing above show the stage of development inside the uterus.
Uterus wall - This is made of muscle. During pregnancy the muscle tissue expands as the embryo grows. It becomes greatly enlarged - for example, a uterus weighing 30g at the start of pregnancy, may weight about 1 kg at the end.
Cervix - A ring of muscle which surroundsthe outlet of the uterus. It is able to expand widely during childbirth.
Amnion - The bag which contains the amniotic fluid.
Amniotic fluid - The water in which the baby floats before birth. It acts as a cushion against shocks and so helps protect the baby from being damaged.
Umbilical cord - This cord links the baby with the placenta. It grows to be about 50 cm long and 2 cm in width. The cord contains blood vessels.
Placenta - A large, thick disc-like structure firmly attached to the wall of the uterus. It is fully formed at about 12weeks and then grows steadily to keep to pace with the baby. When fully grown it is about 15 cm across and weights about 500 g (about 1 lb). When twins are developing, non-identical twins each have their own placenta. Identical twins share the same placenta
Cervical mucus - The mucus forms a plug which seals the uterus during pregnancy.

Function of the placenta:
The placenta is the organ through which the baby feeds and breathes and excretes waste matter while in the uterus. Blood from the baby flows continuously to and from the placenta through the umbilical cord. In the placenta, the baby's blood comes very close to the mother's blood, but they do not mix. However, they are close enough for food and oxygen to pass from mother to baby, and for carbon dioxide and other waste products to pass in the other direction.
Viruses, alcohol, antibodies and chemicals from smoke and medicines, can cross the placenta from the blood of the mother to the blood of the baby. Some of these substances may damage the developing baby, especially in the early months of pregnancy.

Surviving alone: If the fetus is born before 24 weeks it will have a little chance of surviving. The lungs have not yet finished developing and the baby will not be able to breathe properly.
More than half the babies born at 28 weeks survive with expert medical care. If born after 32 weeks, they stand a very good chance, although intensive care will be needed for a while. A baby born before 37 week sis said to arrive pre-term. The usual time (full-term) is about 40 weeks.

Premature babies: A premature baby is one which is born before 37 weeks. Any baby weighing less than 2,5 kg (5 1/2 lb) is also called premature, even if it was born at full term.

Premature baby in an incubator: Premature babies are very small and week and need special care. Frequently they have difficulties with breathing, sucking and keeping warm, and need to be kept in an incubator for the first few days or weeks. The incubator acts as a half-way house between the uterus and the outside world. The baby is kept isolated, protected and in a controlled environment. The temperature is kept constant, so is the humidity. The baby can be fed through a tube or dropper until it has the strength to suck. If necessary, extra oxygen can be supplied to help with breathing.

Miscarriage: The mother has a miscarriage (spontaneous termination) when the baby comes out of the uterus accidentally and too early to survive on its own. The first sign of a miscarriage is bleeding, sometime with pain. Miscarriages are quite common - it is estimated that about 20% of pregnancies and in miscarriage.  The majority of miscarriages occur in the first three months of pregnancy, sometimes very early on, when the woman is not aware that she is pregnant. The usual reason is that there is something wrong with the baby's development.

Ectopic pregnancy: An ectopic pregnancy occurs when a fertilized egg implants itself in the Fallopian tube and grows there. The tube becomes stretched, bleeds and may eventually burst. An operation is needed to remove the embryo and repair or remove the damaged tube. In rare cases of ectopic pregnancy infants have survived long enough to be born alive by Caesarian section.



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