After implantation of the fertilized ovum in the uterus, the embryonic stage begins. This stage, which lasts about two months, sees the development of the organs and systems of the body. This development is a very delicate process, and any damage to the embryo at this time is particularly dangerous and can have irreversible effects. Many factors, such as drugs (thalidomide) taken by the mother, certain infections (such as German measles or rubella) or an X-ray examination can have harmful consequences. In general it can be said that the earlier development is disturbed, the more serious the consequences will be. Contrary to what is often thought, it is wrong to assume that after three months ofthe foetus is no longer vulnerable to potentially harmful influences. Unless drugs are absolutely necessary for the mother they should be avoided during .
At the beginning of the third week the embryonic disc, which formed in the pre-embryonic stage and from which the embryo proper now develops, begins to change rapidly. The disc is made up of three distinct layers of cells. From each of these layers specific parts of the body now begin to form. By about 18 days a furrow becomes apparent, running the whole length of the embryo. This is the neural groove, or canal, which forms from an ingrowth of ectodermal cells. It marks the beginning of the development of the brain, spinal cord and nervous tissue.
Meanwhile, the embryonic disc is changing shape. The growth in length is initially greater than that in breadth, causing the ends to bulge into the enlarging amniotic cavity. This cavity then folds round to enclose the whole embryo in its protective membrane. At the same time, the lateral surfaces of the disc fold inwards. The embryo now becomes pear-shaped, caused by an uneven proliferation of cells in the region that ultimately develops into the head. It is now possible to distinguish between the ends of the embryo for the first time. The head (cephalic) end is the broader of the two and the tail (caudal) is the narrower end.
The pattern of development of the embryo is now established: first the head, then the body, then the arms and legs. This pattern is still evident when the baby is born: compared to adults, the baby’s head is the largest part of its body, and the arms are still as long as the legs.
The first part of the central nervous system to develop is the brain, which appears in the third week as two bulges at the head end. As these bulges increase in size the lips of the neural groove join together at the top edges to form the neural tube within the embryo’s body. It is this neural tube that develops into the ventricles of the brain and the centre of the spinal cord. Lining the tube are ectodermal cells which will form the nerves and brain tissue.
The embryo is developing rapidly, and is dependent for nourishment and oxygen on the mother’s blood. Therefore it is essential that the structures necessary to obtain supplies of these substances develop rapidly too. Within four weeks the embryo has developed a rudimentary cardiovascular system. There are blood vessels in the mesoderm of the embryo itself, in the yolk sac and in the connecting body stalk. The heart is at this stage only a simple U-shaped tube two millimeters long folded into the chest region, but it is already beating. This miniature heart occupies a relatively large space in the embryo, and can be seen as a sizable protrusion in the chest region. Compared to the rest of the body the embryo’s heart is about 10 times as large as in adults.
A circulatory system from the embryo that is continuous with the villi is complete as early as 21 days. The villi are the finger-like projections of the trophoblast that began to grow into the wall of the uterus in the pre-embryonic stage. They too are developing rapidly and in a few weeks will form the placenta. Each villus now contains blood vessels and begins to branch out, forming a tree-like structure. These structures are known as chorionic villi and they penetrate deeply into the lining of the uterus. During this process they open up blood vessels in the uterus, bathing the area in maternal blood. This blood and that of the developing baby are always separate. Nutrients and oxygen pass from the mother’s blood by diffusion* through the walls of the villi into the blood vessels of the embryo via the body stalk. Waste products pass out in the opposite direction by the same method.
By the fourth week the embryo has lost its pear-shape. Head- and tail-folds and then side-folds develop, which close together underneath, making the embryo cylindrical in shape. Enclosed is a space that will become the inside of the gut. The rapid growth of the neural groove results in the long cylindrical shape bending round so that what will be the back of the neck is the uppermost part of the curve. This flexion gives the embryo a rather bean-like appearance because the head and body are more or less of equal size at this stage.
Whereas development is rapid throughout the embryonic stage (from conception until the end of the first month the weight of the embryo increases 10,000 fold) actual growth in size is small. At the end of the first month the embryo is only about 5mm long. In this early embryonic stage it looks very similar to the embryos of other vertebrates. It certainly does not look human yet. The embryo has a tail and there are sulci at the side of the head that look like the gills of a fish. These phenomena are said by some people, including scientists, to reflect our evolutionary development. Of course, later on the tail disappears and the sulci on the head do not develop into real gills, but into muscles of mastication, cheeks and ears.
Health and early pregnancy
During the first few weeks of pregnancy the majority of women experience some degree of physical discomfort, caused mainly by changes in hormonal levels. The expectant mother may feel in turns nauseous, tired and generally moody.
By far the most common complaint is morning sickness, best combatted by eating a light, bland snack (such as biscuits) before getting out of bed and by drinking fluids regularly during the day if vomiting is likely to lead to dehydration. Should symptoms persist a doctor may prescribe antihistamine drugs or vitamin B6. More rarely, vomiting may be severe enough for the woman to be admitted to hospital and put on an intravenous drip to replace lost fluid. Constipation is another frequent complaint of early pregnancy. The remedy is the same as it would be at any other time: plenty of fluids and high-fibre foods, such as fruit. Some women may also find that they need to urinate much more often.
In general, these unpleasant side-effects do not last long and are more irritating than disabling.