Pregnancy is dated from the first day of the last menstrual period. This is an old convention still in place originally based on the thought that conception occurred during menstruation. Although we now know that conception occurs mid cycle, around the time of ovulation, the old system is used when calculating your expected due date. In effect, a woman is not actually pregnant during the first two weeks. A full-term pregnancy lasts 266 days or nine months, based on a lunar calendar of 29 1/2 days. It is usually referred to as 40 weeks, divided into three trimesters of 12 or 13 weeks each. Be assured that a normal pregnancy can last anywhere between 38-42 weeks.
Fertilisation
During ovulation, an egg, or ovum, is released from the ovary and travels down the fallopian tube towards the uterus (womb). Fertilisation occurs when one of the millions of sperm penetrates the outer layers of the egg and fuses with its nucleus.
A sperm penetrating an egg
Genetic Code
Both the egg and the sperm carry 23 chromosomes that pair off to determine every aspect of your baby’s genetic code. This includes sex, eye colour, intelligence, hair colour, height, whether you are having a single baby or multiple birth, among several other defining factors. The cluster of cells, at this point called a morula, rapidly multiplies as it continues its journey towards the womb.