Twins account for over 90% of multiple births and far outnumber triplets or quads in the multiple birth stakes. Currently almost 3% of all the babies born come in multiples.
Identical or monozygotic twins form when 1 single egg is fertilised by 1 single sperm. Almost immediately, the egg splits into 2.
Fraternal or dizygotic twins occur when 2 eggs are released from the ovaries and they are fertilised by 2 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 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 had. It has been common practice for a long time to insert 2 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 that are born very prematurely have a greater chance of survival nowadays.
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.
African women have a much higher chance of having twins. Women of Asian descent have the lowest incidence.
A woman’s chances are increased if she is a fraternal twin herself, or her mother and/ or grandmother were twins, or if she has siblings that are fraternal twins. It is the genetic endowment of ovarian hyper-stimulation on the mother’s side that makes all the difference. Having fraternal twins starts with having 2 eggs being released rather than 1.
Assisted reproduction increases the likelihood of twins. Commonly prescribed fertility drugs stimulate the ovaries to support more than 1 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 urine, 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 1 baby. It makes sense that 2 babies take up double the room. Pregnancy is usually not obvious until after the 12th week when the womb 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 womb much earlier than 1 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 seeing 2 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 ante-natal 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 2 separate heartbeats are heard.
Identical/ monozygotic/ non-fraternal twins
The estimate for identical twins is that around 1 in every 3 sets is identical. They form from 1 egg being fertilised by 1 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.
Though all identical twins are formed very early in pregnancy, there is some variation in exactly how early the fertilised egg splits into 2. This can happen at the two-cell stage on Day 2, the early blastocyst stage on Day 4, or in the late blastocyst stage on Day 6. The stage when the egg splits into 2 will determine where the fertilised eggs will implant in the uterine wall. Once the embryo gets to the 8-cells stage the foetal DNA takes over.
The stage at which the egg splits, will also determine if the babies have their own amniotic membranes and placenta, or if they share. Essentially, the earlier the egg splits, the more independent each twin will be by having their own amnion, chorion and placenta.
Early splitting also means that the twins are perhaps less identical than those who separate at the late blastocyst stage. Around ¼ of all identical twins are said to be mirror images of each other. This means that the left side of one twin exactly matches the right side of the other. How amazing is that!
Although identical twins share the same DNA and genetic coding, they are still unique little individuals. Small differences separate them and while they can appear like little clones of each other at birth, they are far from it. As identical twins grow, their unique qualities (particularly in relation to their personalities) become more apparent. The environment in which they are raised, as well as their own personalities, help to shape the adults they will become. This is exactly the same as any other baby born the world over.
Non-identical/ dizygotic/ fraternal twins
Most twins are non-identical twins; around 2 out of every 3 sets of twins fit into this category. They form when 2 separate eggs are fertilised by 2 separate sperm, so these twins share no more genetic characteristics than normal siblings.
Non-identical twins can be the same gender or different genders. Statistically, the chances are pretty much the same for both.
Fraternal twins are just like any other brothers and sisters in the same family who share the same biological parents. The only difference is that they just happen to have been conceived in the same month and be sharing their mother's womb during the same pregnancy.
Fraternal twins tend to run in families - the tendency for hyper-ovulation during the same menstrual cycle is a genetic trait. For women whose mothers, grandmothers, aunts or sisters had fraternal twins, the likelihood for them to have fraternal twins is higher. Importantly, if a mother is a fraternal twin, she is more likely to have fraternal twins herself.
Fraternal twins have their own little space in their mother's womb and are surrounded by their own amniotic membranes. They do not share a placenta and each has their own. Unlike identical twins, fraternal twins can be different genders or the same: 2 of the same or 1 of each. It seems that the odds are almost equal for fraternal twins to be either the same gender or opposites.
What are the risks of having twins?
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.
If the identical twins share a placenta they are at a higher risk of having twin-to-twin transfusion syndrome (TTTS). This often results in 1 twin gaining more nourishment than the other, and having a higher birth weight as a result. The likelihood is that around 15% of monozygotic twins will have some degree of TTTS.
Umbilical cord entanglement and compression if the twins share an amniotic sac. Purely because of the crowded conditions that occur in the mother’s womb.
Small for gestational age or intra uterine growth restriction. Twins are generally much smaller and lighter in birth weight than singleton babies of the same gestational age.
Conjoined twins – though parents are usually advised of this when there is a diagnosis during pregnancy ultrasound.
With both identical and non-identical twins, there is an increased risk of miscarriage, and pregnancy and birth complications. There is also an increased likelihood of prematurity and associated problems.
Increased risk of needing to have a caesarean section delivery or an assisted delivery, including the use of forceps or vacuum extraction. Mothers, who have a vaginal delivery with twins, generally need to have an epidural and an episiotomy. This is because the birth needs to be controlled by the doctor and midwife so that complications are minimised. Vaginal birth is only considered if the first twin is cephalic (head down) and the second twin is not significantly bigger, i.e. less than 500 grams heavier than the first twin. Twins are however more commonly delivered by caesarean section.
Common twin pregnancy complications for the mother include:
Morning sickness, which commonly extends to Hyperemesis Gravidarum (extreme vomiting)
Premature birth – if a mother can get to 38 weeks carrying a twin pregnancy she is doing very well
Caesarean section delivery – particularly if this is the mother’s first pregnancy and she has not laboured before
Bleeding/ placenta praevia/ placental abruption
There is also an increased risk of developing pregnancy induced hypertension (PIH), polyhydramnios (extra amniotic fluid) and iron deficiency anaemia.
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
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 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 that can then be sent off to the laboratory for testing. Identical twins will share exactly the same DNA but fraternal twins will only share ½. 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.