Twins - identical or non-identical?

What are the different 'types' of twin pregnancies?

If you're pregnant with twins you'll quickly discover there's a lot to get your head around. One of the biggest questions you'll have will almost certainly be whether or not your twins are identical. Twins are either monozygotic (identical) or dizygotic (non-identical). Triplets or more can be monozygotic, dizygotic or a combination of both.

How are identical twins conceived?

Monozygotic (MZ) twins are identical (you may also hear the terms monozygous or uniovular). MZ twins arise when a single fertilised egg (zygote) splits in two at some stage during the first 14 days after fertilisation. The resulting twins have identical genetic make-up and are of the same sex. Identical twins are about half as common as fraternal twins. There are no factors that make you more likely to have identical twins – they're simply a superb quirk of nature.

How are fraternal twins conceived?

Dizygotic (DZ) is the term used for non-identical twins, although you will also hear them referred to as dizygous, binovular or fraternal.

DZ twins occur when two separate eggs are fertilised by two different sperm and there are two separate zygotes (fertilised eggs) from the very start of the pregnancy.

Consequently, DZ twins are genetically no more alike than any set of siblings. About a third of non-identical twins are both girls, a third both boys and a third a boy and a girl.

Interestingly, it is possible for two eggs to be released at slightly different times in the same cycle and be fertilised by sperm from two different men, so you could actually have twins that are only half-siblings. For obvious reasons, this is not very common.

There are several factors that make you more likely to have dizygotic twins. These include:

• Having had hormonal fertility treatment

• Age – older mums are more likely to have fraternal twins

• Number of previous pregnancies – the more you've had the more likely twins are

• Genetics – fraternal twins run in families, but only on the mother's side as it's all to do with hyperovulation – releasing more than one egg at once. Men can pass this gene on to their daughters but men are no more likely to father fraternal twins themselves as they don't ovulate

• Ethnic origin – twins are more common in some ethnic groups than others. They are most common in Nigerian families.

How can I find out if my twins are identical?

Most parents will find out at their 12-week scan whether their twins are identical or non-identical. It's often at this appointment that you first discover you're carrying twins, so it can be a big day, information-wise. If you've been particularly sick or tired, though, you may have had an inkling.

Can a scan tell me if my twins are identical?

The sonographer will have a close look at the placenta and membranes during your scan to try to ascertain whether the twins are identical or not. It's quite a complicated business, all to do with what organs and other pregnancy paraphernalia the babies share. Here's a basic rundown of what they'll look for.

There are two membranes, the amnion (the inner sac, which holds the amniotic fluid) and the chorion (the outer protective sac, from which the placenta grows). By studying these as well as the placenta(s) a sonographer can make certain assumptions. Here comes the science bit:

• If there are two placentas and each baby also has its own amnion and chorion, the twins are known as dichorionic diamniotic (DCDA) – not to be confused with ACDC. DCDA twins can be either identical or not but if they are identical it must mean the fertilised egg split within three days of conception in order to develop two chorions and therefore two placentas. It's more likely twins with two placentas are non-identical but plenty will also be identical.

• If the babies share one placenta and one chorion but each has their own amniotic sac, they are known as monochorionic diamniotic (MCDA) and are always identical. Around 60% of identical twins are MCDA.

• If the babies share one placenta, one chorion and one amniotic sac (cosy!) they are monochorionic monoamniotic and are identical. Only about 1% of twins are MCMA.

Just to confuse things further, sometimes two placentas can fuse together because they have implanted very close to each other in the womb, so it can be quite hard for a sonographer to tell whether there is one placenta or two.

Why do doctors need to know if my twins are identical?

Whether your twins are identical and, more importantly, whether they share a placenta, can affect the type of care you receive and mean you're likely to have more than the usual antenatal appointments and scans during your pregnancy. Antenatal care for twins generally means you’ll be monitored more closely and seen more regularly.

If your twins share a placenta there is an increased risk of complications. It's nothing to panic about, and your doctors and midwives will be well versed in looking out for any potential problems, but this is just one reason why it's important for them to ascertain which things the twins share, or don't. With twins that share a placenta, 15% will develop twin to twin transfusion syndrome (TTTS), meaning one of the twins gets too much blood delivered to it from the placenta and the other doesn't get enough. This can be harmful if not spotted and treated, so you'll be carefully monitored if this it looks like this is a risk. There are treatments available for TTTS, though they do carry risks, and sometimes twins with TTTS are delivered more premature than would normally be advised as they can sometimes be safer out than in.

Are scans always accurate at spotting identical twins?

Usually they get the right answer, but it can be a tricky call. If it looks like there is just one placenta, the sonographer might assume the twins are identical, but it could be that they are fraternal and the placentas have fused. Or they may have two placentas, making it more likely the twins are fraternal, but in fact the zygote split within three days and the babies are identical.

If the sonographer is unable to tell, they may book you in for another scan. This will happen quite quickly as it's much easier to determine whether twins are identical or not before you are 14 weeks pregnant.

Sometimes it isn't possible for them to be absolutely certain, in which case they will look again at your 20 week scan to see if they can tell what sex each baby is. Obviously if they are not the same gender you will know they are not identical.

And a few words of warning to twin parents-to-be: this will by no means be the last time your twins completely fox you – think of it as good practice for all the fun that is to come.

Will I be able to tell if my twins are identical once they are born?

If the 20-week scan doesn't give it away, the doctors and midwives at the birth will examine the placenta(s) more carefully to try to give a firm answer. However, it probably won't be possible to tell just by looking at the babies. As they get older, eye colour, hair, other distinguishing features and the order and timing of their teeth coming through may give it away. It's usually very clear by two years old.

Can DNA zygosity testing tell for sure twins are identical?

Yes. But not until after the birth usually. Finding out whether twins are monozygotic or dizygotic is known as zygosity determination – “telling you how the zygotes 'zygot' there in the first place”, if you like.

Zygosity testing is a DNA test, done by taking a swab with a cotton bud from inside each baby's cheek and sending them off to a lab for testing. The results are more than 99.99% accurate.

The test isn't offered on the NHS but you can pay for it privately. Prices differ but you're looking at somewhere in the region of £100 per twin. Sucks if you've got quints, eh?

Why should I have zygosity testing done?

There are lots of reasons why parents want to have a definitive answer:

• To work out how likely you are to have twins again. If you had fraternal twins, your chances of it happening again are about four times higher

• To satisfy your own curiosity – most parents do obviously want to know

• In case it's ever important for medical reasons – identical twins are much more likely to be useful to each other should one ever need blood, bone marrow or organs. Hopefully that'll never be the case but it's good to know

• To be able to tell other people when they inevitably ask (you will spend at least the first five years answering this question on a daily basis and trust us, you'll get sick of saying 'not sure' pretty quickly. If only because that invites EVEN MORE questions when you just want to fill your Tesco trolley and get out of there before the next feed/change/tantrum.