A new baby is conceived when a woman releases a mature egg at 'ovulation' (about 2 weeks before her next period is due). Her egg is fertilised by one of the many million sperm produced by the man, ejaculated during sexual intercourse. About 3 to 5 days before ovulation a woman starts to produce special, thin, slippery fertile mucous (resembling egg white). This fertile mucous sits in her cervix and lines her uterus and fallopian tubes. If she has sex while this mucous is present, the man's sperm can swim through the mucous, drawing the sperm up into the woman's fallopian tubes, to 'lie in waiting' for her to release an egg. This fascinating process, including the male and female reproductive system, are explained in depth here.
Conception usually takes place in one of the woman's fallopian tubes (depending on which ovary releases a mature egg). When the egg is released into the adjacent fallopian tube, thousands of waiting sperm (that have survived the journey) surround it. A group of the stronger sperm start to burrow with their heads into the egg's thick outer covering (called the 'zona pellucida'). The head of each sperm release a special enzyme (called 'acrosin'), which allows their head to create an opening in the egg's zona pellucida. When the most determined sperm reaches the inner part of the egg, it breaks through into the egg's centre. As soon as this happens a chemical reaction takes place, automatically changing the egg's covering to become impenetrable to any other sperm. At the same time the thrashing tail of the successful sperm detaches, leaving only its head within the egg.
Image 66-07 shows the woman's egg with the head of the successful sperm inside the egg's covering.
About half of a man's sperm carry the genes to form a girl baby (known as the X chromosome) and the other half of his sperm carry the genes to form a boy baby (known as the Y chromosome). Therefore, it is the man who determines the sex of each child. The man's 23 chromosomes in his single sperm (all his genetic coding) and the woman's 23 chromosomes in her egg (all her genetic coding) combine and shuffle both parents' genes to create a unique person with a varied genetic make-up of 46 chromosomes. Nature encourages this 'mixing of different genes' to help the next generation adapt to our changing environment, as the human race evolves.
NOTE: When conceiving twins (triplets or more) the woman usually releases 2 (or 3 or more) eggs at once. These eggs are then fertilised to create non-identical twins (or triplets etc.) The woman's ability to produce more than one mature egg at a time is something that is inherited (and why non-identical twins are more common, because they 'run in the family'). Identical twins only occur after fertilisation takes place. The fertilised egg (with all the combined genes from both parents) suddenly splits into two identical developing babies. This is a twist of fate that happens less frequently, but is possible for any couple, because it is not something that is inherited. You can read more in twins, triplets or more - the pregnancy.
When the sperm enters the egg they fuse and combine their genes to make one cell. The fertilised egg is then known as a 'conceptus'. Within hours of the conceptus forming it splits into 2 identical cells.
Image 66-02 shows the 'conceptus' after splitting into 2 cells.
Hours after the conceptus splits into two cells, they split again to create 4 cells.
Image 66-03 shows the 'conceptus' after splitting into 4 cells.
This process of creating new identical cells continues, so that by the 4th day after fertilisation the conceptus becomes a group of about 16 to 20 cells, called a 'morula' (or a 'ball of cells').
Image 66-23 shows the 'morula' as a ball of cells.
On the 5th day after fertilisation, most of cells in the morula start to move to one side, creating a bunch of cells inside a sac of fluid (that the baby will float in). The individual cells (which all previously looked the same) now start to change (or 'differentiate') as they begin preparing to perform their individual functions, now destined to become the different parts of the baby's body (and the placenta). As cell differentiation continues, the 'morula' becomes what is known as the 'blastocyst' by day 6 after fertilisation.
While the conceptus is growing into a blastocyst it is also gently being moved down the woman's fallopian tube towards her uterus. This is achieved by hundreds of small, hair-like structures called 'cilia' (that line the tube), in combination with muscle fibres in the walls of the tube, producing wave-like contractions. By the time the conceptus becomes a blastocyst (taking about 6 days) it enters the woman's uterus to implant in its thick lining (about a week before the woman's next period would have been due).
Image 65-07 shows the egg being released from the ovary and being fertilised by the sperm in the fallopian tube. The conceptus changes into a morula and then a blastocyst, before implanting in the lining of the woman's uterus.
The blastocyst takes another 4 to 6 days to fully embed or 'implant' in the woman's endometrium. The implantation process takes about 6 days to complete (finishing about 12 days after fertilisation, or about 2 days before the woman's next period would have been due). Once the blastocyst is fully implanted, the mother's body starts to provide the developing baby with nourishment to continue to grow. Up until this time the baby has been nourished by glucose secreted from the lining of the fallopian tubes and the uterus.
Read more about early pregnancy