Twins can be identical (from one fertilised egg that splits) or non-identical (from two separate eggs). Identical twins are 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.
An ultrasound scan is the most likely way of discovering you're carrying twins. Even if you haven't had a scan, few women reach labour without knowing they're carrying two babies. You're normally a lot bigger and routine antenatal care means the midwife is likely to hear two heartbeats.
Many mothers expecting twins say it's harder and more tiring. You have a greater load to carry - not just an extra baby, but an extra placenta and extra amniotic fluid too. The added weight means backache and fatigue are more likely. Your blood pressure will probably be higher too.
You may need to rest more often and you may want to think about stopping work sooner.
Although there's a higher chance of twins being born early, most are born only a little before their due date, at about 37 or 38 weeks.
The birth of twins usually goes smoothly, with both born head down. However, caesarean sections are more common with twins. One or both of the babies may end up in a less than perfect position as that's the only way they can make room.
One or both might be breech, coming bottom or feet first. Occasionally, one baby lies across the uterus, in which case a caesarean is essential. Twins can also be smaller and weaker than single babies and a caesarean might be the easiest, least stressful way for them to be born.
Because twin pregnancy and births have the potential to be more challenging, you'll see an obstetrician more often and all aspects of your pregnancy, labour and birth will be carefully monitored.
Twins are more likely to need some time in special care after the birth. You'll probably be advised to stay a little longer in hospital yourself, even if your twins are perfectly healthy (as most are).
This will give you the chance to become more comfortable with feeding and build your confidence about their routine, day-to-day care.
Your body can make as much milk as your babies need - twice the stimulation of the supply, means twice the amount made.
It's the practical aspects that might be more tricky - how to hold them so you're all comfortable, whether to feed them one at a time or together and so on. The babies may not have the same appetite and they may be hungry at different times, too.
You'll need plenty of help with the other aspects of the babies' care, such as bathing, nappy changing and washing clothes.