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Fertilisation is when the sperm meets the egg. I'm sure you know all about the things that lead up to that occasion.

The erectile tissue within the penis is filled with blood as the man becomes sexually 'aroused'. The erect penis can then be placed into the woman's vagina.

During sexual intercourse the penis is moved back and forth until ejaculation occurs. This reflex involves muscles around the sperm duct squeezing outthe semen in a series of contractions.

The semen gets fired up inside the vagina. From then on they are on their own - well all 500 million or so of them.

The sperm swim upstream against a flow of fluid moving back down the oviducts and uterus. They head towards the egg but many millions never make it. Only about 100 sperm make it to the egg. All the others perish. But 'there can only be one'!


These 100 sperm battle away at the outer coat of the egg until finally one manages to break through. It enters the egg and immediately fertilises it. This triggers a change in the outer coat which stops any other sperm getting in.

So only 1 sperm fertilised the egg out of 500 million. Not great odds for a sperm!

The fertilised egg continues its journey down into the uterus. During which time it divides a number of times becoming a ball of cells.

At this stage this zygote is renamed an embryo.

fertilised egg

It becomes embedded or 'implanted' into the lining of the uterus. This is where the baby will develop over the next 9 months.

As the embryo grows it becomes surrounded by a bag called the amnion. Inside this bag is amniotic fluid, this cushions and protects the embryo.

embryo grows

The embryo is supplied with food, water and oxygen via the umbilical cord that attaches it to the placenta. Waste materials such as carbon dioxide are also removed.

The placenta is a wonderful structure that is attached to the wall of the uterus and allows a very close meeting between the baby's blood and the mother's.

The two blood streams don't mix but molecules diffuse across a thin barrier between them.

It's the first question everyone asks.

But the real question is what determines the sex of the baby?

Of the 23 pairs of chromosomes in the human cell's nucleus there is 1 pair that are called the 'sex chromosomes'.

In women these chromosomes are the same size and are called X chromosomes. However men have one X chromosome and a smaller one, the Y chromosome.

So we could say that a woman has the genotype 'XX' while a man has 'XY'. (The phenotypes are what the genes actually produce, the external features).

X and Y chromosomes

Now the tricky bit! You knew that there had to be one.

When the gametes are made through meiosis all the chromosome pairs are split up. But since both of the woman's sex chromosomes are X, each egg cell will contain an X. However men's sperm could either have an X or a Ychromosome.

Another way to show that is in a 'Punnet square' diagram.