Fertilisation

The two routes to understand are the use of fertility drugs so that conception may occur inside the body, and in vitro fertilisation (IVF), in which fertilisation takes place in a laboratory.

When the nuclei fuse at fertilisation, the fertilised egg receives one set from each parent and has two sets.

In sexual reproduction, two gametes join at fertilisation. A body cell is usually diploid: it has two sets of chromosomes, with one chromosome of each pair inherited from each parent. A gamete must instead be haploid, carrying just one set.

Different methods interrupt reproduction at different stages: they may prevent ovulation, stop sperm reaching an egg or make fertilisation very unlikely. This means that methods cannot be compared only by asking whether they are “hormonal” or “non-hormonal”; their mechanism, reliability, risks and suitability also matter.