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Identical twins occur when one fertilised egg splits into two. Each baby will have the same genes and will be the same sex and look very alike.
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 be of the same sex or different sexes, and they will probably look no more alike than other brothers and sisters. A third of all twins will be identical and two thirds non-identical.
Signs that you might be carrying twins
You might suspect you are carrying more than one baby if:
- you are very sick in early pregnancy
- you seem bigger than you should be at your stage of pregnancy
- you have had fertility treatment
You should find out if you are carrying more than one baby at your dating ultrasound, which happens between eight and 12 weeks.
You will need another scan after this to find out whether the babies share a placenta (are identical), or if they have two separate placentas, in which case they could be either identical or non-identical.
It is important to know this because women with babies who share a placenta will need to have more appointments and scans. If it cannot be determined, you should be offered a further scan.
A third of identical twins have two separate placentas. This happens when the egg splits in the first three to four days after conception and before it implants in the uterus.
How multiple pregnancies differ
Multiple pregnancies have a higher risk of complications, in particular, premature birth.
If your babies share a placenta (identical twins), or if you are having more than two babies, you should be offered your pregnancy care at a special clinic for multiple pregnancies. These clinics are not held in every hospital in Northern Ireland and you may have to travel to another hospital.
Your doctor might also refer you to the Regional Fetal Medicine Service in Belfast if you require more specialist care.
If your babies share a placenta or if they are triplets, it is recommended that you are scanned every two weeks from 16 weeks onwards and every four weeks if your babies have separate placentas.
You are more likely to have a caesarean section, but twins can sometimes be delivered vaginally. You should discuss this with your doctor.