LMC Care

Early ultrasound scans for determining chorionicity

By Dr Emma Parry, Multiples NZ Medical Advisor

There is an increased maternal and fetal risk in twin pregnancies, more-so if it involves monochorionic fetuses. Chorionicity (how the membranes are set up) is determined by ultrasound—and the earlier the better, and definitely before 16 weeks! After this time, the babies are too big to accurately determine the chorionicity.

If an ultrasound is performed early enough in the pregnancy, the yolk sac (which disappears gradually as an embryo develops) may be visible on the scan. It is worth trying to find out if there is one sac or two, as some research suggests that there is a direct relation between the number of amniotic and yolk sacs.

Generally, if there are two yolk sacs, then there will be two amniotic sacs. If a woman has been diagnosed as carrying monoamniotic twins, but saw two yolk sacs on an early scan, then there is the possibility that she has been misdiagnosed. It is reason enough to continue looking for a membrane dividing the embryos, unless and until she has separate confirmation of the diagnosis.

The amniotic membrane can be very thin... so thin, that it's extremely hard to see on an ultrasound, making misdiagnoses common. If the diagnosis was made before 8 weeks, it is definitely not certain: it is simply not possible to get a reliable determination of whether a membrane is present or not before 8 weeks. The best time to check for the membrane is between 10 and 12 weeks, and again between 16 and 20 weeks.

At these times the membrane is usually easier to see. If you are diagnosed before that time, you may want to ask for a repeat ultrasound during one or both of these time-frames. Once you are referred to an Obstetrician, you are likely to have fortnightly ultrasounds, which should ensure you have at least one scan during these times.

If you only discover that you’re carrying twins later in the pregnancy and the sonographers can’t tell whether there is a membrane present, there is still a good chance that the membrane was hidden at the time of the scan.

Finally, if an ultrasound reveals the presence of tangles in the cords of both babies, that is fairly determinative of the babies sharing a single sac. Even if there is a membrane, because it didn't prevent the cords from becoming entangled, it is safest to manage the pregnancy as if the babies are monoamniotic.

The first scan, if done prior to 15 weeks, will have nearly 100% accuracy in determining chorionicity. It is standard practice for sonographers to comment on chorionicity at the time of an early scan. If a scan at less than 15 weeks confirms a twin pregnancy but does not clearly state chorionicity, it should be repeated or reviewed.

Nuchal translucency is as accurate in twin pregnancies as in singletons and can provide individual risk of aneuploidy (a type of chromosome abnormality) for each fetus. Any discordancy in nuchal translucency, however, can also be due to an increased risk of development of Twin to Twin Transfusion Syndrome (40% PPV – positive predictive value) or due to other structural abnormality.

If you are diagnosed with carrying monochorionic (MC) twins, whether diamniotic or monoamniotic, you will be classed as having a high-risk pregnancy. In these circumstances you should be managed by either a specialist LMC, or with regular obstetric specialist input, and can expect to undergo regular ultrasound scans.

The following scans are recommended for MC twins:

Dating scan (with determination of chorionicity)
Nuchal translucency scans
Fortnightly scans from 16 weeks gestation for growth and liquor to detect early signs of TTTS.
Where any of these scans indicate any of the following situations, a specialist with an interest in high-risk pregnancy should be involved:
Discordant nuchal translucency
Liquor discordancy
Growth discordancy of greater than 20% EFW (estimated fetal weight).

More about different types of twins

Fraternal twins

Fraternal (dichorionic) twins twins share the same prenatal environment. They each have their own genetic make-up, so they may not be more alike than any other brothers and sisters in the same family. Despite their differences they will share the special bond of being born on the same day and growing up together.

Fraternal twins are more common if the twins’ mother: has a history of twins in her family; is over 35 years old; has already had several babies; and/or, has been taking fertility drugs.

Identical twins

Identical, or monochorionic, twins occur in 25% of twin pregnancies. The latest research indicates that identical twins have a very similar genetic makeup (sharing about 88% of their DNA), resulting in both children looking alike. Identical twins are the same gender, may have similar finger prints, ear shapes, eye colour, hair colour and teeth imprints. They will often develop at similar rates and experience developmental stages simultaneously, for example, when they learn to walk and grow their first teeth. They may also have similar brain wave patterns. Mirror image identical twins have mirror image features, such as left and right hair crown swirl, left- or right-handedness and similar moles or body marks, but on different sides of the body.

What causes a fertilised egg to divide and create identical twins is still unknown.

Identical twins and chorionicity

Identical twins are categorised by chorionicity—depending on when the fertilised egg splits and how it splits, such as top/bottom or right side/left side. After an egg is fertilised it will develop a yolk sac (important for nourishing the forming embryo) and around the same time, the embryo’s placenta and chorionic sac begin to form. Finally, an amniotic sac surrounds the developing baby. There are different types of chorionicity for identical twins, depending on the timing of the afore-mentioned events.

Diamniotic dichorionic twins

If the egg splits before the placenta has formed, within three days of fertilisation, each baby will have their own placenta, chorionic sac and amniotic sac.

Diamniotic monochorionic twins

If the egg splits after the placenta has formed, after the third day following fertilisation, then the babies will share a placenta and chorionic sac, but will have their own amniotic sac. Monochorionic twin pregnancy is more common where assisted reproductive technology has been used.

Monoamniotic monochorionic twins

Monoamniotic twins occur when the egg splits after the amniotic sac has begun to form, around nine days after conception. As a result, both babies share an amniotic sac. Many monoamniotic twins lie very close to, or on top of, each other and early ultrasounds cannot tell if they are separate babies.

Monoamniotic twins are often misdiagnosed, especially at early scans. They are higher-risk than other diamniotic twins, but babies do survive. If you want to know more, the Monoamniotic Monochorionic Support Site, at http://www.monoamniotic.org, is an amazing support network and has a bulletin board with mo-mo mothers around the world.

Conjoined twins

If the egg splits later than 12 days after fertilisation, and does not split completely, then conjoined twins occur.

Signs & Symptoms of Premature Labour

Premature Birth

Maternity care during a multiple pregnancy

By the time you read this, you may have booked in with a Lead Maternity Carer (LMC)—a midwife, GP or obstetrician—and found out that you are expecting multiples, and that your maternity care requirements will be different to a singleton pregnancy. Multiple pregnancy is considered high risk, and comes under the “Transfer" category of the guidelines which cover maternity care. Therefore, your midwife or GP must recommend you be referred to a specialist, and the responsibility for your care will be transferred to this specialist. If you already have an obstetrician, there will be no change to your care.

A woman expecting multiples should be referred to an obstetrician as soon as possible, so that potential complications can be considered and assessed. It is very important that the transfer of care happens promptly, so do not accept delays! This is particularly important if you are having monochorionic twins (identical), or triplets, since complications such as Twin-to-Twin Transfusion Syndrome can start as early as 16 weeks. Once referral has occurred, there will be a discussion on your care options for the rest of your pregnancy.

There are several possible options for specialist obstetric care for a multiple pregnancy—the availability of these options may be dependent on where you live in New Zealand. If you are in a provincial area, you may need to travel to a larger centre in order to get appropriate care. Specialist obstetric care options include:

A hospital obstetrician in the public hospital-based obstetric service, with the hospital’s team of midwives providing antenatal and postnatal care
A private obstetrician
Shared care, with your midwife LMC working in partnership with a hospital-based obstetric-led service or private obstetrician.

No matter which service you choose (or is available), you can expect a high standard of care, increased monitoring (scanning) and a partnership approach to ensure that you receive the best care possible.

Mixed feelings?

Often, when a multiple pregnancy is diagnosed, you will have already found an LMC that you feel comfortable with; you may feel upset about your care being transferred to a specialist, and a change in role for your midwife, especially if you have already developed a good relationship with yours. You may feel uncomfortable with the idea of increased monitoring and worried about unnecessary interventions, or that your beliefs and expectations about a “normal” pregnancy and natural childbirth are now not possible.

One of the most important pieces of advice we can give you at this point, is to keep an open mind about the pregnancy and birth, and to always bear in mind that the health professionals providing your maternity care, whether they are obstetricians or midwives, have the same objective: two healthy babies and a healthy mother. As there are many factors that determine the type of pregnancy and birth you will have, taking a flexible approach might save you from having unmet expectations that leave you feeling disappointed or unhappy. So, plan for what you want, but expect the unexpected!

Important points

Once you are diagnosed as carrying multiples, your LMC should refer you to an obstetrician, as soon as possible. If you develop serious pregnancy complications, such as TTTS, your obstetrician should refer you to a Maternal Fetal Medicine specialist. This should be done immediately. If you are at a smaller hospital you may be referred onto one of the three Fetal Medicine Units in New Zealand (go to www.nzmfm.health.nz for more information.)

If your babies are born before 35 weeks gestation, your care will be managed by the hospital team (i.e. obstetricians, registrars and midwives), irrespective of who your LMC is. If you have a private specialist, he/she will head the hospital team.

Finally, if you are not happy about your care for any reason, talk to your LMC or obstetrician about your concerns. If you need support with this, phone your local multiple birth club or the Multiples NZ for advice. If you cannot resolve your concerns and are still unhappy with your LMC or obstetrician (and you have the option), change your LMC or obstetrician. Phone your local multiple birth club for the names of midwives or obstetricians that others have found supportive during multiple pregnancy.

Problems finding a specialist or LMC

If you are having problems finding a specialist, your local Hospital must provide care—your GP can refer you to the required service. You can also contact the Ministry of Health to find a local LMC: phone 0800 MUM 2 BE (0800 686 223).

For more information on your maternity care options, click here.

For information on the Legislation governing maternity care for multiple pregnancy, click here.

For more information about talking to your LMC or obstetrician, click here.

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