Expecting twins? Here are 9 types you should know about

Around one in every 60 or so births worldwide is a twin birth, yet most of us only know two types: “identical or fraternal”. In fact, there are several more. Here is what every parent should know.
The two you will meet most often

Fraternal twins are the most common kind, making up around seven in every 10 twin pregnancies. They begin as two separate eggs, each fertilised by its own sperm. Genetically, they are just ordinary siblings who happened to share a womb, so they can be two boys, two girls or one of each, and they may look nothing alike.

Some things make a fraternal pair more likely. A family history of twins, being older when you conceive, fertility treatment such as IVF, and even your body type, diet and background can all play a part. The common thread is that the body releases more than one egg in a single cycle.

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Identical twins start as one egg and one sperm. Early on, that single embryo splits in two, and each half grows into a baby. Because they share the same genetic blueprint, they are always the same sex and usually look strikingly alike, with matching skin tone, hair colour and features.

They make up about a third of twin pregnancies, and unlike fraternal twins, they seem to be down to pure chance. Age, family history and fertility treatment do not appear to change the odds.

Identical does not always mean indistinguishable, though. Where each baby sat in the womb, childhood illnesses and later lifestyle habits can all nudge which genes are switched on. That is why many grown-up identical twins end up with slightly different faces, builds and personalities.

A twist on identical: mirror image twins

Here is one that parents love. Roughly one in four identical pairs are “mirror image” twins. If the embryo splits a little later than usual, the babies have already started to work out their left and right sides, and those sides can end up flipped.

You might see one twin who is right-handed and the other left-handed, or a dimple on opposite cheeks. Hair swirls may spin in opposite directions, and birthmarks can appear on opposite sides. They are still counted as identical, since their genes are the same. It is simply a matter of timing.

The rare ones

Conjoined twins happen when an embryo starts to split but does not finish the job, leaving the babies joined at some part of the body. It is very rare, affecting only one or two births in every 100,000, and it is seen more often in girls.

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Doctors describe each pair by where they are connected. Pregnancy carries higher risks, babies are often born early, and delivery is always by caesarean. Whether separation surgery is possible depends entirely on how the twins are joined and what they share.

Parasitic twins are closely related and rarer still, at around one in a million births. One twin stops developing properly and remains attached to the other, who may share organs with it. There is no known cause beyond random chance. As with conjoined twins, doctors decide on surgery case by case, with the healthy baby’s wellbeing as the priority.

Superfetation sounds almost unbelievable. A woman who is already pregnant releases another egg, which is fertilised and begins a second pregnancy, so the babies are conceived at different times. The body normally switches off ovulation once pregnancy starts, so this is thought to happen perhaps once in several million pregnancies.

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Fewer than 10 cases have been reported worldwide, and some experts wonder whether a double uterus might make it more likely. It can also be tricky to tell apart from a regular twin pregnancy.

Heteropaternal superfecundation happens when two eggs are released in the same cycle and fertilised by sperm from two different men. The result is fraternal twins with different fathers. Only a handful of cases have ever been confirmed, usually when a paternity test throws up a surprise. Bollywood fans may remember the 2024 comedy ‘Bad Newz’, which is built around this very plot.

The newest discoveries

Semi-identical twins were first described by researchers in 2006. These twins match completely on their mother’s side, but share only half of their father’s genes. A second case, reported in 2019, was spotted when twins who shared a placenta, which usually points to identical twins, turned out to be a boy and a girl at the 14-week scan. The leading theory is that two sperm fertilise one egg, and the egg then splits.

Polar body twins, sometimes called half-identical twins, are a theory rather than a proven fact. The idea is that an egg divides in two and each part is fertilised by a different sperm, giving twins who share around 75 percent of their genetic markers. That would put them somewhere between fraternal and identical. At present there is no confirmed case, and no test exists to check for it.

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Which kind are yours?

If you are curious, a zygosity test (a simple DNA test) can tell you whether your twins are identical or fraternal. Your midwife or consultant may also be able to tell from your scans, particularly if the babies share a placenta.

(Disclaimer: This article is for general information and only and should not be treated as medical advice, diagnosis or treatment)