Identical or monozygotic twins form when one single egg is fertilised by one single sperm. Almost immediately, the egg splits into two.
Fraternal or dizygotic twins occur when two eggs are released from the ovaries and they are fertilised by two sperm. In terms of similarities, fraternal twins don’t share any more characteristics than siblings with the same biological parents.
The likelihood of twins being boys or girls tends to be fairly equal, with around 50% of either gender being conceived. This matches the incidence of singleton (one baby) conception.
Contrary to public opinion, the likelihood of having twins does not skip a generation. The odds of having twins are the same for every mother in every generation.
Why are there so many more twins around these days?
One of the major reasons is because of assisted fertility techniques and the number of couples who are undergoing reproductive assistance. It was far more common in the past for infertile couples to not have an option about having children. But our current understanding and management of reproductive technology means that couples with fertility problems now have a better chance of conceiving than they ever have. It has been common practice for a long time to insert two or more embryos to increase the chances of a positive pregnancy.
The other reason is that the care of premature babies has become so much better. There is an increased risk with multiple pregnancy of premature labour. Even babies who are born very premature have a greater chance of survival.
What will increase the odds of having twins?
- As well as reproductive assistance, increasing maternal age is another important factor. It is well known that the incidence of twins increases in women who are over 35 years. This is because the hormone oestrogen increases as women mature. This important sex hormone plays a vital role in stimulating the ovaries to release eggs each month. Another reason is because follicle stimulating hormone (FSH) tends to surge in the later years, leading to hyper stimulation of the ovaries.
- How many pregnancies a woman has had previously is a factor in her conceiving with twins.
- Women from Nigerian and African/American heritage have much higher chances of having twins. Women of Asian descent have the lowest incidence.
- If a woman is a fraternal twin herself, or her mother and or mother’s mother were twins, or she has siblings who are fraternal twins, then her chances are increased. It is the genetic endowment of ovarian hyper stimulation on the mother’s side which makes all the difference. Having fraternal twins? starts with having two eggs being released rather than one.
- Assisted reproduction increases the likelihood of twins. Commonly prescribed fertility drugs stimulate the ovaries to support more than one ovarian follicle to maturity. The more eggs available to fertilise, the greater the likelihood of conception occurring.
- Making sure you’re doing all that you can to conceive in the first place. Tracking your ovulation and menstrual cycles and having a lot of sex over the days when you are fertile will certainly help.
How will I know if I’m having twins?
You may suspect you are having twins long before they are positively diagnosed. Extreme nausea can be one of the first symptoms and not being able to keep any food down. It’s as if you can tick all the boxes for early pregnancy symptoms but increase them tenfold. Smells are more acute, your bladder won’t seem to hold any reasonable volume of wee, you’re absolutely exhausted and your breasts may seem to have taken over your chest. This is because the pregnancy hormone, hCG is being produced in mega quantities and its effects on your body have become very evident.
You are also likely to “show” much sooner than if you are only carrying one baby. It makes sense that two babies take up double the room. Pregnancy is usually not obvious until after the 12th week when the uterus starts lifting up out of the pelvis. But with twin pregnancies, a little round bulge is often visible long before the end of the first trimester.
The other give away sign is lots of movements, felt earlier than normal. Quickening – which is the official term for foetal movements – isn’t generally felt until around 16 weeks by first time mothers and as early as 14 weeks for those who’ve been pregnant before. But with twins, those tiny first movements can be very clear. Twins fill up the uterus much earlier than one baby and are in contact with the uterine wall as they bounce and flit around.
You may also find out from having an ultrasound and see two babies on the screen. Sometimes pregnant mothers don’t know they’re having twins until the 18 week screening ultrasound; others as early as 12 weeks. It’s possible to see on ultrasound at 12 weeks or earlier which type of twins a woman is carrying. Unfortunately, identical twins are at a higher risk of having congenital abnormalities and complications, such as being small for gestation age. Fraternal twin pregnancies generally only require normal antenatal care but still need careful monitoring especially when the mother becomes very large and the risk of early delivery increases.
Another way to detect a twin pregnancy is through Doppler; when two separate heartbeats are heard.
Identical twins/monozygotic twins/non fraternal twins
The estimate for identical twins is that around one in every three sets is identical. They form from one egg being fertilised by one sperm and because of this, each twin shares exactly the same genetic material. They look the same and can be very difficult to tell apart.
We don’t really know what it is that makes the embryo divide so early after fertilisation. This process is not well understood and although many theories have been put forward, there is no absolute explanation for why identical twinning occurs.
Non identical twins/dizygotic twins/fraternal twins
Most twins are non identical twins; around two out of every three sets of twins fit into this category. They form when two separate eggs are fertilised by two separate sperm so these twins share no more genetic characteristics than siblings with the same parents.
Non identical twins can be the same gender or different genders. Statistically, the chances are pretty much the same for both.
What are the risks of having twins?
There is a greater risk of miscarriage and other pregnancy complications. The “safest” twin pregnancy to have is considered to be the fraternal twin variety. This is because each twin will have its own placenta and amniotic sac.
Common twin pregnancy complications for the mother include:
- Morning sickness which commonly extends to hyperemesis Gravidarum (extreme vomiting).
- Gestational Diabetes.
- Premature birth. If a mother can get to 38 weeks carrying a twin pregnancy she is doing very well.
- Caesarian section delivery – particularly if this is the mother’s first pregnancy and she has not laboured before.
- Bleeding/placenta praevia/placental abruption.
Common twin complications for the baby include:
- Smaller birth weight.
- Problems with feeding.
- Longer stay in hospital after birth.
- Occasionally the babies have problems maintaining their body temperature and stabilising their blood sugar levels.
- Infections and vulnerability due to their size.
- Breathing problems.
When is it clear if twins are identical or not?
It’s not always evident at birth whether twins are identical or fraternal. Though it is uncommon, some identical twins can still be born with their own membranes rather than sharing an amniotic sac. Fraternal twins usually have a placenta each and non-fraternal/identical twins share the same one.
In order to be 100% sure a DNA test can be done. This is a straightforward way of ensuring what type of twins the babies are. Obviously if they are different genders there’s no point, but lots of parents of twins choose to have the testing done. All it entails is an inner cheek swab which (painlessly) collects some cells which can then be sent off to the laboratory for testing. Identical twins will share exactly the same DNA but fraternal twins will only share half. Another way to test is to do a blood test for grouping. If the twins have the same blood group then they are more likely to be identical.
Last Published* April, 2023
*Please note that the published date may not be the same as the date that the content was created and that information above may have changed since.