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Identical twins are the result of one fertilised egg splitting into two separate cells. Each cell grows into a baby. Because they originally came from the same cell, the babies have the same genes. They are the same sex and look very like each other. Non-identical twins are more common.

They are the result of two eggs being fertilised by two sperm at the same time. The babies may not be the same sex and will probably look no more alike than any other brothers and sisters.

Twins happen naturally in about once in every 65 pregnancies. A couple is more likely to have twins if there are twins in the woman's family. Triplets occur naturally in one in 10,000 pregnancies, and quads are even rarer. Nowadays the use of drugs in the treatment of infertility has made multiple births more common.

You may suspect that you are carrying twins if:

• you are very sick in early pregnancy
• you seem bigger than you should be for your dates
• twins run in your family, or
• you have had fertility treatment

It is usually possible to find out through an ultrasound scan by the end of the second month of your pregnancy. You should be told at this point whether the babies are identical or non-identical. If this can't be determined, you should be offered a further scan.

All multiple pregnancies have a higher risk of complications, particularly premature birth. If your babies are identical it is recommended that you are scanned every two to three weeks from 16 weeks onwards and every four weeks if your babies are non-identical. You may be advised to have a caesarean section and you should discuss this with your doctor, but it is your choice. It is possible to breastfeed twins and even triplets. You may find that a combination of breast and bottle feeding is best for you, particularly if you have triplets or more.

Find out more in our ‘breast feeding and bottlefeeding’ section.