How are twins formed?
Identical twins (or monozygotic twins)
- Identical twins happen when a single sperm fertilises an egg, and then, at a very early stage, the fertilised egg divides into two and starts forming two babies.
- Identical twins have the same genes, so they’re the same sex.
- Some of these twins have their own separate placenta (afterbirth) and sac to grow in the mother’s uterus (womb) but many share the same placenta and a few also share the same sac.
- Monozygotic twins might be quite different sizes at birth, but they become more alike with time. They’re often hard to tell apart when they’re older.
- Identical twins do not usually ‘run in families’ and it is not known why monozygotic twins occur.
- Identical twins happen about once in every 250 live births.
Non-identical twins (or dizygotic twins)
- Non-identical twins are also sometimes called fraternal twins. They happen when two separate eggs are fertilised by two different sperm so that two embryos (the beginnings of a baby) are formed.
- Each has its own separate place in the uterus and separate placenta and sac.
- They might be the same sex or different sexes.
- Dizygotic twins are often similar at birth, but they become less alike as they get older, as do other (different age) brothers and sisters.
- Dizygotic twins are more likely to happen when there are twins in the mother’s family, or if the mother has been having fertility treatment. If a mother is a non-identical twin, she has about a 10% chance of having twins herself. (A mother of twins who is not a twin herself has about a 5% chance of having another set of twins.) If the father is a twin, this doesn’t make it more likely that the parents will have twins.
- About two births in every hundred are dizygotic twins. There has been a rise in the number since the use of fertility treatment has become more common.
Siamese twins (or conjoined twins)
- Siamese twins are rare.
- They’re twins who come from the same egg (monozygotic) but the embryo doesn’t separate completely to form two separate babies.
- This means that the babies are joined together in some way. Some can be separated by an operation without too much difficulty. In other cases they share vital organs and cannot be separated without the death of one or both twins.
A vanishing twin
With ultrasound being done early in pregnancies, it has been found that many more pregnancies start as twin pregnancies (up to 5% at 12 weeks), but one of the babies stops developing. The other baby develops normally.
Knowing if twins are monozygotic or dizygotic
Usually your doctor will work out whether your twins are monozygotic or dizygotic soon after the birth. If they’re different sexes they’re definitely dizygotic. Monozygotic twins are more likely to have some physical problems at birth, and also more likely to have similar health problems when they’re older – so it can be useful to tell if they’re identical or not. Since monozygotic twins can look very different at birth, the doctor might have the placenta very carefully examined, or do blood tests.
Pregnancy and birth with twins
- Some of the problems (complications) with pregnancy, such as ‘morning sickness’ and weight gain might happen earlier with twins.
- Twins are usually born earlier than single births, and the babies are usually smaller. You can read more about premature birth.
- The babies might be born vaginally, especially if both babies are in a head-down position, but a caesarean section might be a safer way of delivery for some twins.
- There are more health risks for newborn twins, but usually the risks are similar to the risks for single babies of the same weight.
- Because of their premature birth, some twins develop breathing problems, and will need special care or intensive care for a while.
- Sometimes one monozygotic twin will be much smaller than the other, because the placenta wasn’t equally shared. This baby might have some health problems at birth, but usually the smaller baby will have caught up in size within six months.
Preparing for twins
It is important to prepare your mind as well as your body for the twins’ arrival.
- Once you know you are having twins it is a good idea to start thinking about them as separate individuals so you can get to know them.
- You can start building your relationship with your babies by getting to know their movements and their position in the womb. You can also use ultrasound pictures to share your experiences with the babies’ father.
- You might feel emotionally and physically drained by the changes happening to your body. It’s important to share these feelings with your loved ones. This helps them to start sharing the care.
- Read as much as you can about twins and how to care for them.
- If you get offers of help – accept! You might feel uncomfortable about this at first. Every bit of help you get will help you to build a better relationship with your babies. People like to be helpful. You could, for example, accept help with cleaning, ironing, shopping or with preparing food.
- Freeze some meals in advance for times when you need them after the birth.
- In the first few weeks while you’re getting settled you might need a lot of help. This is a good time to plan for your partner, a family member or family friend to be available to help out.
- Plan what you’re going to do about nappies. A nappy service or disposable nappies might be helpful.
- Because twins often come early, plan to go to your antenatal classes a bit earlier than usual to make sure you can complete them. Your midwife or doctor will be able to help you with this planning.
- If you have other children at home, particularly very young children, think about how you’re going to prepare them for the new babies.
Because breastmilk supply increases with extra demands, most women can breastfeed twins well. This can mean much less work compared to preparing and giving formula.
- Because of the extra challenges of feeding two babies, it might help if you get in touch with your local breastfeeding association, a lactation consultant or your local branch of the Multiple Birth Association before the birth of your babies. Your midwife or community child health nurse will also be able to give support.
- Breastfeeding has many advantages. It provides the best food for babies and it helps prevent some common infections.
- Make sure you get as much rest as you can and remember to eat nutritious meals.
- You can feed the babies together or separately. Feeding both babies at once can give you more time to get some rest in between feeds. You might want to feed separately some of the time so you get more chance to get to know each baby.
When you can, it’s a good idea to feed each baby separately, because this separate closeness and touching helps encourage bonding with the baby. If you try to feed them together you’ll find you’re holding the bottles, not the babies!
Caring for twins at home
If you can manage getting a bit of extra assistance in the house after the babies are born this can be a great help – even if you have to go without something else.