THE BEGINNING OF
A NEW LIFE
Conception:
For a baby to be conceived, you must have intercourse around the time of ovulation. In a regular menstrual cycle, this fertile time occurs about midway between periods. It means that an egg (ovum) is released by one of the two ovaries, which are lined with the womb (uterus) by two Fallopian tubes.
The egg moves down the tube ready for fertilisation. This happens if one of the millions of male sperm present in an ejaculation of semen intercepts the egg and successfully penetrates it. Then egg and sperm unite to form a single cell which contains an equal complement of genes ('building blocks' of heredity) from each parent.
These genes, which have come together to create a unique new baby, are arranged along 23 pairs of chorosomes - the DNA chains which carry the genetic code in every cell of our bodies. One member of each pair is contributed by each parent.
So, at the moment of conception, the new combination of genes will ready to transmit chemical messages to decide whether your baby will dark or fair, , tall or short, male or female, and which parent he will most resemble in facial appearance. (Because brown is 'dominant' over blue in genetic terms, a born to one blue-eyed and one brown-eyed parent is likely to have brown eyes.)
Boy Or Girl?
One factor in a baby's development in which the mother's genetic contribution has no influence is in the matter of sex determination. Whether your baby will be a boy or girl depends on whether the paternal sperm which fertilised the egg contained a female X-chromosome or a male Y-chromosome.
Of the 23 paris of chromosomes which the baby inherits from both parents, one pair consists of two sex chromosomes. The mother, who has two female X-chromosomes, contributes one of these to the baby. The father, who has one X-chromosome and one Y-chromosome, may contribute one of either of these to produce a girl or boy baby.
Occasionally, parents would like to be able to choose the sex of a child. At present, there is no reliable method of gender selection available to healthy parents. Besides, most doctors would reject the idea of such non-essential interference with nature, on ethical grounds.
However, where a couple have a high risk of passing on a life-threatening hereditary condition affecting children of one sex (sually boys), it is now possible to ascertian the sex of the fertilised egg by means of 'test-tube baby' technology.
This means that only fertilised eggs which are known to be free of the risk of inherited illness are placed in the womb for development. Needles to say, this very difficult procedure is never likely to be available on a large scale.
How twins are conceived?
Between one in 80 and one in 100 conceptions result in the birth of twins. Your chance of having twins increases if there is a history of twin births in either parent's family, in which case the babies are more likely to be non-identical (fraternal) rather than identical twins.
Identical twins (always of the same sex) result from the fertilised egg dividing in two at conception. Each baby has the same arrangement of genes, which accounts for physical similarity. Both babies share the same placenta, but each has its own umbilical cord and protective amniotic sac ('bag of membrances').
Fraternal twins - non-identical twins who are no more alike than other brothers and sisters - may be of the same of different sex. They are conceived independently of each other when, as occasionally happens, two eggs are released simultaneously by the ovaries and are fertilised by two different sparm they each have a seprate placenta as well as separate cords and amniotic sacs.
