How Fertilisation Works

By Dr John Kennedy

Fertilisation occurs when a woman’s egg and a man’s sperm fuse to form a single cell.  This usually occurs in one of the fallopian tubes.  For this to happen, the egg and the sperm have to perform certain functions beforehand.  Once fused, the merged cells must find their way to the uterus and embed into the lining of the womb in order for the fertilisation to become a pregnancy.

Here is a brief overview of the roles played by the female egg and the male sperm:

The Female Egg

The exact time of the month for ovulation depends on your menstrual cycle.  Taking an average menstrual cycle of 28 days, ovulation occurs on days 12-15.  Day one is the first day of your period.

At ovulation, an egg is released from the ovaries.  It is picked up by and travels down one of the fallopian tubes towards the uterus where, if intercourse has taken place within the last four days, it may meet sperm.

Eggs live and can be fertilized for 12-24 hours after being released.  Sperm a lie and stay active in your body for up to 48 hours.

Hormones prompt an increase in blood supply to the womb in preparation for implantation.  It takes up to five days for the fertilised egg to reach the womb and embed itself in the lining.

If the egg is not fertilised, or if the fertilised egg cannot attach to the womb lining, then a period begins.

The Male Sperm

At the point of ejaculation during intercourse, a man can release up to 300 million sperm into his partner’s vagina.  Only a small proportion of those make it through the neck of the womb and on to the fallopian tubes.  The sperm must be actively moving, of normal appearance and of sufficient quantities to be considered normal.  It must also be capable of moving through the fallopian tube, where the egg is fertilised.

The quality of the cervical mucus in the woman’s body at the time of ovulation must be such that it allows free passage of the sperm into the uterus.

Finally, only one sperm will find its way to fertilise an egg.

The Fusion of the Female Egg and the Male Sperm

Once the sperm penetrates the egg, the chromosomes carried by the sperm and the egg come together and the egg is fertilised.  Within hours, the microscopic zygote divides over and over to produce multiple cells.  Over the period of about 5 days, the fertilised egg or embryo now made up of about 150 cells, makes its way to the uterus or womb.

At this point, if conditions are favourable, the embryo embeds itself in the lining of the womb and the female becomes pregnant.