Types of twins

There are two types of twins – identical and non-identical. There are two membranes surrounding all babies when they are inside the womb

– the outer membrane, which is continuous with the placenta, is called the chorion and the inner membrane is the amnion. In most pregnancies the two are so close together they are indistinguishable from each other.

Non-identical twins are due to two eggs being fertilised and have two separate placentas (although they may be beside each other). Each baby grows in its own sac, is swimming in its own amniotic fluid and is surrounded by its own membranes (the amnion and the chorion) – this is called a dichorionic diamniotic pregnancy.

Identical twins grow from one fertilised egg and the type of pregnancy depends upon the age (in days from conception) at which the pregnancy split into two. This means that identical twins can be:

  • Dichorionic and Diamniotic (just like non identical twins)
  • Monochorionic and Diamniotic (the babies share one placenta and one chorion but each have their own amniotic membrane and are in their own amniotic fluid)
  • Monochorionic and Monoamniotic (the babies share one placenta and one set of membranes – this means the babies are together in one sac and share the same amniotic fluid)
  • Conjoined, or Siamese (these are extremely rare)

We can usually tell from your early ultrasound scans whether your twins are Dichorionic and Diamniotic (in which case they could be identical or non identical), Monochorionic and Diamniotic (identical) or Monochorionic and Monoamniotic (also identical). This is important because some risks associated with identical twin pregnancies are dependent on the arrangement of the placenta, membranes and amniotic fluid.

Nausea and vomiting in early pregnancy

Unfortunately multiple pregnancies can cause nausea and vomiting in early pregnancy that is more severe than with single pregnancies. Please see my general pregnancy information sheet about this.

Metabolic demands

There is little doubt that twins place a greater strain on the maternal metabolic and nutritional system. For this reason I recommend that women carrying twins take a daily iron and folic acid supplement such as Fefol or FGF. These supplements are best absorbed if they are taken with orange juice and if tea and coffee are kept to a minimum. Unfortunately iron supplementation can cause or exacerbate constipation and it can darken your bowel motions.

Growth of your babies

For any time after approximately twenty weeks of pregnancy, twin babies tend to be a little smaller than single babies. The reasons for this are complicated and not well understood but probably include nutritional factors, genetic factors, competition for placental blood supply and simply running out of room. After twenty weeks I recommend monthly growth ultrasound scans to monitor the growth and wellbeing of your babies.

If one or both of your babies is growing slowly I will organise to carefully monitor their wellbeing. If we are concerned about the wellbeing of one or other of your babies we may need to consider delivering them early.

With identical twins that are Monochorionic both babies are sharing the same placenta. Sometimes blood vessels connect from one side of the placenta to the other in such a way that blood can flow from one baby to another. This is calledTwin to Twin Transfusion Syndrome, or TTTS. When TTTS arises one baby (called the recipient) gets more blood than the other baby (the donor). Too much amniotic fluid can surround the recipient, while the donor can have very little amniotic fluid around it. TTTS often arises quite early in pregnancy (sometimes before twenty weeks).

In order to look out for signs of TTTS I request fortnightly ultrasound scans for women with Monochorionic identical twins. If we detect signs of this syndrome arising at an early stage of pregnancy we will discuss how to manage the situation, as there are a number of effective interventions for this problem, including laser of the placental vessels responsible.

Premature labour and birth

You are more likely to go into labour or give birth early with twins than with one baby. Again the reasons for this are not clear but include simple mechanical issues (such as the womb being bigger than usual for any given stage of pregnancy). Tragically, twins are occasionally – but rarely, fortunately – born before they are able to survive outside the womb in a neonatal intensive care unit.

You should watch out for the symptoms of premature labour, which include:

  • Regular painful contractions or tightening of your womb (Braxton Hicks contractions, which are common and not of concern tend to be painless)
  • A feeling of heaviness in your lower abdomen or lower back discomfort (both being like the time just before your period)
  • Pinkish fluid, water or blood coming out of your vagina
  • Constant lower abdominal pain

Please contact me directly if any of these symptoms occur.

There is some scientific evidence suggesting that performing ultrasound scans of the cervix (or neck of the womb) can help us to anticipate premature labour. The best way to do this is by a vaginal scan where a small ultrasound probe is placed in the vagina and the cervix is closely examined. I request this type of ultrasound scan at approximately 18, 24 and 28 weeks, although can be done in conjunction with ultrasounds for other reasons and is not a separate visit.

If you have significant symptoms of premature labour and / or your cervix is changing at a premature stage of pregnancy I am likely to admit you to hospital for rest and observation. While it is often not possible to stop babies being born prematurely, there are some things we can do to try to slow the process down or, if your babies are to be born early optimise their condition.

Another reason that twins are more likely to be born earlier than single babies is that we might intervene to deliver them early because of a complication of the pregnancy. The most common reasons for doing this are slow growth and concerns about the wellbeing of one or both of your babies (see above) or pre-eclampsia (high blood pressure in pregnancy, see below).

Medical complications of pregnancy

Medical and midwifery students are all taught that with twin pregnancies all possible medical complications of pregnancy are more likely with twins (and more likely yet with triplets!) than with single pregnancies. This relates to the size of the pregnancy and in particular the combined size of the placentas. I have mentioned some of these complications (slow growth and premature labour) above.

There are two other important complications of pregnancy that are more common with twins than with single babies – gestational diabetes and pre-eclampsia.

Placentas secrete a number of hormones that increase maternal blood sugar levels. While this has the obvious benefit that it gives the baby a steady supply of sugar to use for growth and development, sometimes the mother’s blood sugar level can be a little too high. This effect can be more marked in twins simply because of the increased size of the placentas. This is why I will organise for you to have a pregnancy diabetes screen (PDS) blood test at around 26 weeks to check that you have not developed gestational (pregnancy – related) diabetes. If you do have diabetes I will refer you to a physician colleague to discuss monitoring and managing your sugar levels. Good control of your sugar levels can often be achieved by diet and exercise so insulin injections are only rarely required. Gestational diabetes – especially when brought on by a twin pregnancy – almost invariably resolves as soon as your babies are born.

Pre-eclampsia is high blood pressure that arises during the pregnancy. It is usually associated with protein in the urine (this is why I check your blood pressure and urine at each visit) and it has a number of significant potential complications for mothers and their babies. Pre-eclampsia is more common with multiple pregnancies. The cure for pre-eclampsia is for your babies to be born although it can sometimes take a week or so for your blood pressure to return to normal after the birth. If you develop significant pre-eclampsia and your babies are mature I am likely to advise organising their delivery. On the other hand if pre-eclampsia arises at a premature gestation I will recommend hospital admission, close observation and delivery if the condition of you or your babies’ becomes worrying.

Full term” for twins

Technically at least a twin pregnancy is “due to deliver” at the same time as a singleton pregnancy. However, as mentioned above, twins often come early and many obstetricians view “full term” for twins to be three weeks earlier than for singles. This view is supported by a number of scientific studies that suggest that the risks for twin babies beyond 38 weeks are the equivalent for a single baby one or more week overdue. For this reason I generally suggest delivery of your babies at 38 weeks or so, depending upon your wishes. If you are not keen on intervention we can consider the alternative of keeping a close eye on your babies with heart monitoring tests and ultrasound scans.

Labour and birth of twins

Labour and birth with twins is often very straightforward, particularly if both babies are head first. However most obstetricians recommend a caesarean delivery if the first baby is breech (or bottom first) and some recommend a caesarean if the second baby is breech.

I advise continuous monitoring of your babies’ heart rates during labour. I suggest that you have an intravenous drip in your arm in case there is bleeding after the birth. An epidural has some theoretical advantages in a twin labour (in addition to making the pain go away!) but this is something we should discuss individually.

There are some theoretical risks with vaginal birth for twins – particularly for the second twin. Sometimes the second baby can manoeuvre itself into an awkward position following the birth of the first baby and very rarely its umbilical cord can enter the vagina before the baby. One large Scottish study suggested an increased risk of problems with the birth of second babies such that the alternative of delivering twins by caesarean should at least be considered. I will discuss this with you and we will reach a mutual decision as to the most appropriate way for you to give birth. Your wishes, the position of the babies and the presence or absence of the medical complications affecting the pregnancy will help us to decide.


I suggest you read as much as you can about twins, because their birth is only the beginning of a very interesting journey…