The development of a human individual fromup to the time of birth, takes on average 266 days. Midwives and doctors talk about the duration of pregnancy in weeks, or even in months. This seems to be less accurate than measuring in days. But as pregnancy is a natural phenomenon there is a great individual variation. Delivery up to two weeks before or after the calculated day can be considered as normal. When talking about the duration of pregnancy, it seems logical to start from the moment of conception. This moment, however, is not normally known, so calculations are usually made from the date of the last menstruation. As conception usually takes place about two weeks after a menstruation, in medical language it is common to say that pregnancy lasts 280 days, which is equivalent to 40 weeks or just over 9 months. It would be very confusing to talk about the third week of pregnancy, when in reality has just taken place. Gestation can be divided into three stages: the pre-embryonic, the embryonic and the foetal. The pre-embryonic stage begins with fertilization and lasts for about 14 days, during which time the fertilized ovum consists of a mass of dividing cells which have already begun to differentiate. In this stage implantation of the fertilized ovum in the uterus also takes place.
The embryonic stage from the fourteenth day to the third month is the time during which all the main organs of the body are formed. By the end of this stage the embryo has a recognizably human shape and is now called a foetus. During the foetal stage, from the third month until birth, the newly-formed body develops fully and grows. Finishing touches such as toe nails and finger nails form, until a new individual is ready to be born and to survive independently.
Each new human life begins with the fusion of two cells – a sperm (spermatozoon) from a man with an egg (ovum) from a woman – to form a single cell. This process of fusion, called fertilization, is the culmination of sexual intercourse when 200 million sperm are ejaculated into the vagina. Only a few hundred of these sperm will complete the journey from the vagina through the cervix and uterus to the Fallopian tube, where there is a possibility that just one of them may manage to fertilize the egg. Abnormal sperm tend not to survive this rather rigorous journey, thus decreasing the chance of abnormality in the new individual. To achieve fertilization the sperm has first to penetrate the loose layer of cells, the corona radiata, that surrounds the egg. The sperm achieves this by producing enzymes* which dissolve part of the transparent layer around the egg. Once these barriers have been broken, the outer membrane of the head of the sperm can fuse with the thin outer membrane of the egg. The sperm head is then engulfed (the tail is left outside) and the egg immediately reinforces the barrier at its surface to prevent more sperm from penetrating it. Within a few minutes the centre (pronucleus) of the egg fuses with that of the sperm. This is the actual moment of fertilization.
Sperm can survive in the woman’s reproductive tract for about three days. Fertilization, however, can only occur on two days of a woman’s monthly cycle. This is the time immediately after ovulation, when the egg is shed from the ovary and is present in the Fallopian tube. If the egg is not fertilized, it passes to the uterus and dies.
Each sperm and each egg contain 23 chromosomes, half the number found in other cells in the human body. Fertilization brings these together to make a complete set of 46 chromosomes at which point the genetic make-up, physical characteristics and sex of the new individual are decided. These characteristics are derived from both the mother and the father. However, it is in effect the father who determines the sex of the child.
When the pair of sex chromosomes divides, the mother’s (XX) can, in the egg, only be an X; but the father’s (XY) can in the sperm be either X or Y. Y-bearing sperm produce boys, X-bearing ones, girls. Once fertilized, the egg – or zygote as it is now called – passes slowly along the Fallopian tube to the uterus. This takes about three days and is assisted by contractions of the tube (peristalsis*) and by fine ‘hairs’ called cilia on the cells lining the tube which waft the zygote along.
As the zygote travels it divides into two, then four, then eight new cells, and so on. Before each cell division all the chromosomes are copied so that each new cell again has a complete set.
This small, solid ball of cells somewhat resembles a mulberry in shape and is called a morula. As development proceeds there is a build-up of fluid inside the morula, between the cells, and a cystic structure known as the blastocyst is formed. When it reaches this stage it enters the uterus, some five to seven days after fertilization.
Inside the uterus
Once inside the uterus the blastocyst moves around freely for about two days, the cells growing and dividing rapidly, the whole unit is able to increase in size now it is out of the confines of the Fallopian tube. The cells also begin to differentiate.
The blastocyst consists of an outer layer of cells known as the trophoblast and a cluster of cells called the inner cell mass from which the embryo develops. On about the seventh day a critical stage in the nutrition of the blastocyst is reached when it begins to run out of sufficient nutrients. The early nourishment of the morula and blastocyst is by diffusion from the tubal and uterine fluids but this has now become inadequate. It must therefore embed itself in the wall of the uterus to obtain a new source of nourishment from the mother. This process of embedding is known as implantation which completes the first stage of pre-embryonic development.