Conception, fertilization and implantation 

How natural conception takes place

Most women know that at ejaculation, a man deposits millions of spermatozoa into the vagina. The vast majority of these will not make it past the womb. Only a few thousand get to where the egg (ovum) is, which is usually at the outer end of one of the fallopian tubes. Only one of these will actually fertilize the egg. The sperm and egg fuse to form one cell, called a zygote. 

This cell will undergo division and vast changes in the following days, resulting in a mature baby made up of billions of cells; most of them specialized for a particular function about 40 weeks later. 

What happens after fertilization

Things are happening quietly. There is absolutely no way for a woman to tell immediately whether a sexual act has resulted in conception. All those popular novels in which a woman says that “I knew immediately that I had conceived” are sheer fantasy. 

Within thirty hours of fertilization, the fertilized egg undergoes the first division to form two cells. This is only the beginning of billions of divisions and changes that will take place within the first few weeks of pregnancy.

The dividing ball of cells – which is still quite tiny in size – is, meanwhile, being swept steadily along the tube towards the cavity of the womb, where, all being well, it will implant.

Site of implantation

After the tubal journey, lasting 4 to 5 days, the ball of cells, now numbering 50 to 60, reaches the uterine cavity. Implantation commences around seven days after fertilization and is complete by day 14.

Evidence of implantation

In most cases there is no sign or symptom that herald implantation. In a few cases, there may be light vaginal bleeding. Since this occurs about three weeks or so after the last period – and the woman is still unaware of her pregnancy – this light bleeding may be mistaken for a menstrual period. It has been known to cause confusion in dating the pregnancy. However, this problem is easily sorted out using ultrasound scan, provided this is carried out early, that is, within the first half of the pregnancy and ideally within the first trimester.

The serious business of turning this mass of cells into a human being starts after implantation. Even though it is still tiny in size, organs start forming at three weeks and their formation is complete by eight weeks. 

Beyond this point, only maturation and increase in size is taking place. There is no new organ formation.

Even though the fetus is only about 2.3 cm in length (less than one inch) at eight weeks, the nose, ears, finger- and toe­-buds are already formed.