Foetus

3D image of human foetus at three monthsA baby in a mother’s womb is called a foetus after three months of pregnancy, and appears like a perfectly formed but tiny baby. Before this it is referred to as an embryo.

The foetus floats in a fluid filled sack-like a water filled balloon. It drinks the fluid, and excretes into it through its kidneys and bowels.

One side of the balloon is a special outgrowth of the baby, which forms the placenta, while the rest is a fine but tough transparent membrane. The baby is connected to the placenta by the umbilical cord, which at birth is between 15 and 120 cm. long, and runs from the navel to the centre of the placenta. The arteries and veins in the placenta fan out and penetrate into the wall of the uterus to interact with the mother’s circulatory system. This enables the baby to draw oxygen and food from the mother’s system, and send waste products to the mother for removal.

At 16 weeks, the foetus is 12 cm long and its sex can be determined. The skin is bright red because it is transparent, and the blood can be seen through it. The kidneys are functioning and producing urine, which is passed into the amniotic fluid.

The “quickening” is the time when the mother becomes aware of the baby’s movements. It occurs between 16 and 18 weeks (the latter in first pregnancies). The mother usually becomes quite elated at this time, as she realises that there really is a baby inside her. The movements become gradually stronger throughout pregnancy, until it is possible to trace the movement of a limb across the belly. Babies vary dramatically in how much they move – some are very active indeed, while others are relatively quiet. During the last couple of weeks of pregnancy the baby does not move as much, as the amount of space available becomes more restricted.

By 24 weeks, the skin is the normal colour. This is the earliest that a baby has a reasonable chance of surviving outside the mother, although infants are still at high risk if born before 32 weeks. By that stage, development is complete, and the last eight weeks are merely a growth stage.

By 38 weeks, the baby has settled upside down in the uterus. During this period, the head sinks down into the mother’s pelvis and is said to “engage” ready for birth.

The miracle is completed when labour starts. The trigger for this is not accurately known, but a series of nervous and hormonal stimuli dilates the cervix that guards the opening into the womb, and starts the rhythmic contractions of the womb, which will bring another human being out into the world.

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