Double trouble
Twins can be identical (from one fertilised egg that splits) or non-identical (from two separate eggs). Identical twins have to be the same sex, as they have the same genes. Non-identical twins can be the same sex or one of each.
Fertility treatments make twins more likely - maybe 25 per cent of in vitro fertilisation (IVF) conceptions result in twin births. Some other treatments stimulate ovulation, which increases the chance of more than one egg being released and fertilised.
The increase in multiple births due to fertility treatments has caused concern among doctors. Some feel the practice of replacing two embryos after IVF should stop.