One egg is released every month, but many others are wasted at the same time.

Every month one egg is released from the ovaries, alternating between left and right each month. If that egg is not fertilised, the lining of the womb is shed and the egg simply degenerates and is lost.

Although only one egg is released from the ovary, many eggs begin the maturation process along with it. A wave of around 10-15 eggs start to grow every month but only one becomes the dominant egg. This dominant egg continues to grow until it overtakes the rest and is finally released at ovulation. All the others which were not strong enough to become the dominant egg are now lost from the pool. This means that every month each ovary loses many eggs, with all but one being wasted. A woman begins her life with hundreds of thousands of eggs in her ovaries, but after many waves of eggs growing and degenerating every month, they quickly deplete and eventually run out completely resulting in menopause.

IVF works by rescuing the eggs which would usually be lost.

To create embryos outside the body, eggs need to be extracted from the ovaries so they can be fertilised in the laboratory. Hormonal stimulation is used to take control over egg development in the ovaries. Rather than having one dominant egg, the smaller ones are allowed to grow simultaneously alongside it, rescuing the eggs which would normally be lost that month.

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