While many mums won't know that they are carrying twins until this first trimester nears its end, and although twins and other multiples grow and develop at about the same rate as single babies at this stage, a multiple pregnancy is different to a single pregnancy right from the start. Here's what is happening in the first trimester.
How it all begins
There are two main kinds of twins: fraternal and identical. Fraternal twins and higher multiple pregnancies come about when more than one egg is fertilised and then implant successfully in the uterus. These twins are really just like any other siblings except that they are the same age as each other. These days the majority of fraternal twins are the result of fertility treatment, which raises the chances of multiple pregnancy as several fertilised eggs are usually transferred to the womb together. But fraternal twins can also occur naturally when the ovaries release more than one egg and both are fertilised. A predisposition for fraternal twins can run in families, which isn't the case for identical twins.
Identical twins, which are less common than fraternal twins, are the result of a single fertilised egg splitting into two identical parts shortly after conception and continuing to grow as two separate embryos. Because identical twins come from a single sperm and egg, they both share the same genetic information but this doesn't mean that they will look, and be, exactly the same. Identical twins each have their own distinct personalities and, in fact, can have markedly different temperaments and character traits.
Early development in the first trimester
In the first few days after conception, your fertilised egg/s, now bundles of cells now known as morulas, travel down your fallopian tubes and into the uterus where they burrow into the lining of the womb. Usually, half of the cells attach to the womb lining and start to form the placenta, a baby's support system throughout pregnancy, and the other half of the cells start to form the embryo. Fraternal twins always each have their own placentas and their own sacs of amniotic fluids, and can implant in quite different parts of the womb. Identical twins, however, can sometimes share a single placenta and, although very rare, they may even share a single sac of amniotic fluid.
For the first twelve weeks of pregnancy your growing babies are still embryos and are undergoing the most critical time of development. The early miscarriage rate for multiple pregnancies is higher than that for singletons, though a miscarriage won't necessarily affect both babies if it does occur and an expectant mum may not even be aware of it happening early in pregnancy: Scans very early in pregnancy have shown that sometimes one twin is miscarried, and is absorbed back into the body, while the other twin develops healthily; this is known as 'disappearing twin syndrome' and will only usually be detected if the mother has had a scan showing a twin pregnancy in the first few weeks.
Although many women still don't even realise they are pregnant by this stage, never mind expecting two babies, by the end of week four most twins will be visible on an ultrasound scan as two gestational sacs. An ultrasound after week six may detect your twins' heartbeats as separate rapid blinkings on the screen. Of course most women don't have their first scan until the end of the first trimester, but if you have had fertility treatment or experience problems with bleeding early in a pregnancy then you're more likely to have a scan in these early weeks.
Throughout the first trimester and up until nearly the end of the second trimester, around 24 to 26 weeks, twin babies both grow and develop at about the same rate as single pregnancy babies. (For ultrasound pictures and a week-by-week account of how your babies grow in the first two trimesters take a look at the links at the bottom of this article.) By end of week twelve - around the time that most mums to be see their babies on ultrasound for the first time, and so often when you first realise you're expecting twins - your babies will both span around 7 - 9 centimetres and weigh between 14 and 20 grammes.
Although there is still an enormous amount of growth and development to come yet, all your babies' essential organs will now be present and your two bundles of cells will have transformed themselves into something more recognisable as two mini human beings complete with head, arms, hands, legs and feet.
What's happening with mum
Because your body is doing twice the work of single pregnancies it's common to be particularly tired when carrying twins, and in fact this is often one of the first clues to a multiple pregnancy: Tiredness is a very common pregnancy sympton in the early weeks of pregnancy, as your body adjusts to being pregnant and puts so much energy into this critical developmental stage, and with twin pregnancies the tiredness can be extreme. You may also feel other pregnancy discomforts, such as morning sickness more severely because you are producing higher quantities of pregnancy hormones that can cause the nausea.
Another early sign that you may be carrying twins is a bigger increase in appetite than with single babies. Whereas a woman carrying a single baby will need about 300 extra calories a day, you'll need double that - 600 extra calories. Your figure will probably have rounded out at least a little by the end of the first trimester and you may find that some of your clothes are already getting a little uncomfortable, this usually happens a bit faster for mums expecting twins than with singleton pregnancies.
Of course, if this is your first pregnancy then you won't have anything else to compare this pregnancy with, and in any case you won't necessarily feel any different, or any extra discomfort, with a multiple pregnancy.
If you've only just found out that you're expecting twins then it might take you and your partner some time to adjust to the idea. You'll probably have lots of questions about what happens next and caring for your pregnancy from here on in and can find many of the answers in articles such as our overview of pregnancy with twins, pregnancy complications and discomforts with twins and eating well when you're expecting twins.
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