The third month of intra-uterine life is concerned mainly with growth and refinement of the main organs and systems of the body, which have already formed during the previous two months. The embryo, now definitely human in appearance, becomes known as a foetus.
At eight weeks the foetus is still only about 3cm long but by 12 weeks it is about three times this size. A plentiful supply of nutrients is necessary to sustain such a fast growth rate and the placenta, essential for the foetus to obtain all it requires, completes its development during this time. The nutrients are, of course, supplied by the mother through the maternal blood supply to the placenta. They include amino acids and simple sugars such as glucose, which undergo biochemical conversion – that is, they are metabolized – within the growing foetus to supply it with the bodybuilding proteins and energy.
The placenta began to develop in the second week when the finger-like villi first burrowed into the wall of the uterus. This was the first stage of uteroplacental circulation. By the twelfth week the now-branching chorionic villi closest to the source of maternal blood
have formed the placenta, which is discoid in shape and made from both foetal and maternal tissue. The villi contain foetal blood vessels which join together and are linked to the foetus by the umbilical cord. The cord, which develops from the body stalk, is the life-line of the foetus. It is composed of gelatinous tissue (Wharton’s jelly) surrounded by membrane and contains one vein, which carries oxygenated blood full of nutrients to the foetus, and two arteries which return the blood containing waste products to the placenta. (This is the opposite arrangement from the normal circulation, in which arteries carry oxygenated blood from the heart, and veins carry deoxygenated blood back to it.)
Substances cross the placenta, which is in effect a selective barrier, mainly by simple diffusion through the walls of the villi, with no mixing of maternal and foetal blood. Some larger substances, such as amino acids to build proteins, are pushed across the barrier by special transport processes. Large substances such as bacteria generally cannot cross the placental barrier, thus protecting the foetus from their noxious effects. Some smaller harmful organisms, however, such as the rubella (German measles) virus can get through and cause abnormalities in the foetus. That is why immunization against rubella is recommended for girls in their early teens, before they are likely to get pregnant. An abortion may be recommended for a woman who contracts rubella in the early stages of.
An important function of the placenta is the production of. Only a few days after implantation the production of the hormone* human chorionic gonadotropin (HCG) is started. This hormone induces the yellow body of the ovary (corpus luteum) to continue secreting oestrogen and progesterone. These hormones are vital for pregnancy because they stimulate the development of an adequate surrounding for the embryo by maintaining the endometrium, the lining of the womb, and by suppressing further ovulation in the ovary. Progesterone inhibits contractions of the uterus and thus prevents the foetus from being expelled. In the third month the placenta itself is able to produce oestrogen and progesterone in increasing amounts. The yellow body of the ovary now becomes redundant.
Thus the placenta acts as lungs, stomach, liver, intestine and kidneys for the foetus. In fact the only vital process carried out by the foetus itself is the maintenance of its heartbeat and circulation.
The facial features become more defined; the forehead broadens and the facial muscles develop, filling out the face. The eyes continue to move forwards, almost reaching their correct forward-facing position although they are still wide apart. The lens in each eye is almost completely developed by 12 weeks and the membrane from which the pupil develops, and the retina at the back, are both present. The eyelids grow down and join together, remaining closed until the seventh month. The ears continue to move up to their final position on the side of the head and most of their external features are well developed. The inner part of the ear begins to develop now, too. The nose and mouth are almost complete. The nose is divided into two internal cavities by a membrane; the mouth is separated from the nasal cavity by the palate and contains dental buds that will form both milk teeth and permanent teeth.
The urinary and digestive systems develop further in the third month. The kidneys start to function as the foetus swallows amniotic fluid and excretes it as urine, although the placenta remains the main excretory organ. The digestive system, at first a simple tube, now forms into distinct regions. The oesophagus stretches from the mouth to the stomach bulge and a large loop of the midgut or intestine, which was contained in the umbilical cord, is withdrawn into the foetal abdomen. The hindgut forms the colon and rectum, opening at the tail end of the foetus to form the anus. The liver and pancreas are also well-formed in the upper part of the abdomen, and the pancreas begins to secrete the hormone insulin, which controls the metabolism of glucose, the chief energy source for foetal growth.
The windpipe (trachea) grows downwards from the mouth and branches to form the intricate air passages within the developing lungs. The foetal heart, formerly a simple single-chambered pump, develops four chambers but still pumps mainly a mixture of oxygenated and deoxygenated blood. The various parts of the brain also begin to form and take shape.
By the end of the third month the sex of the foetus can be distinguished by external appearance. In the second month both sexes developed similar external genitalia: a phallus and folds or swellings. Now the male foetus secretes male sex hormones from his testes, which cause the phallus to grow into a penis and the folds and swellings to form the scrotum. In the female foetus the ovaries secrete female sex hormones and the folds or swellings develop into the labia majora and labia minora. The phallus remains small and becomes the clitoris.
The human appearance of the foetus is further advanced in this month. There is a pronounced change in its proportions, particularly in the relative size of the head and body which at eight weeks are almost equal. Now the body grows more rapidly than the head.
One external refinement is the beginning of nails on fingers and toes, which now appear as a thickening of tissue at the tip of each digit. The limbs generally develop further giving greater definition to joints and more flexion.
The skeleton, which formed from soft tissue, now starts to turn to bone (ossify). Calcium is deposited in the tissue which turns first to cartilage and then to hard bone. The hardening process starts in distinct regions called ossification centres and spreads outwards. At first, plates of cartilage with a long bone grow and generate more cartilage. The `bone’ thus increases in length. At ossification centres at the ends of the bone and at the centre of the shaft the cartilage hardens into true bone. Later the inner central bone is supplied with blood vessels and turns to fatty marrow. The calcium for the formation of bone has to come from the mother, which is why foods rich in calcium (such as milk, cheese, eggs and fish) are an essential part of a pregnant woman’s diet. The brain and spinal cord become more complex as the week pass, while the continuing development of nerves and muscles enables the foetus to make movements, although they are still too small to be felt by the mother. They include movements of the arms that bring the hands into contact with the face, and movements of the chest that resemble respiratory movements.