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MS WordPerinatologist Corner - C.E.U/C.M.E. Modules
Twins: Antepartum assessment and Intrapartum management
Sponsored by The Indian Health Service Clinical Support Center
2. Twin Pregnancy: Double Trouble?
Step 2. Background and Initial Work-up
Case Study
Mrs. BP is a 21 year old primigravida who presents for her first prenatal visit at 10 weeks by her dates. She is found to have a 14-15 week size uterus, but fetal heart tones were hard to find. An ultrasound is obtained and reveals a twin pregnancy. A membrane is noted between the two fetuses. She is really excited, but a bit overwhelmed by this news. The first thing she wants to know is if her twins will be identical.
- What ultrasonographic signs would you look for to try to answer her question about chorionicity? (or is it zygosity??) Why is it important to know?
- What would you counsel her about her risks now that she is known to be carrying twins? Are you comfortable managing, or co-managing, twins at your facility?
BACKGROUND
Multifetal births account for about 3% of all births, but the incidence is increasing, especially of higher order multiples, as a result of assisted reproductive technology. (Hall) Multiples account for almost 1 in 5 preterm births, and their perinatal morbidity and mortality is doubled compared to singletons. Twins alone account for 10% of the total perinatal mortality. The poor perinatal outcome is the result of prematurity, fetal growth restriction, and anomalies. (Rosello-Soberon) On average, twins are born at 35 weeks, triplets at 32 weeks,
and quads at 30 weeks. We won’t spend much time discussing higher order multiples beyond twins here as, in general, these are women who should be referred for more specialized care
at the time of diagnosis. (Dodd ) (Luke)
ZYGOSITY OR CHORIONICITY?
Dizygotic twins
account for about 70% of all twins. They are formed from two fertilized eggs, and are genetically distinct. They are sometimes referred to as fraternal twins. Two placentas develop, each with chorion and amnion, and they are thus dichorionic, diamniotic. Because the placentas are separate and there are usually no vascular connections between them, the problems caused by the fetuses sharing a circulation are not an issue. This type of twins usually has fewer complications and better outcomes. (Shvieky)
Monozygotic twins result from a single fertilized ovum that subsequently divides, usually at the blastocyst stage. They are thus genetically identical. The usual case is for there to be a single placenta with two amnions, making them monochorionic, diamniotic. Between 10-15% of monochorionic placentas will have arterio-venous connections that can result in twin to twin transfusion, a serious problem with a high mortality rate that we will discuss a little later.
Only a very small percentage of twins (0.4-1.4%) are monochorionic, monoamniotic.
Such twins are always monozygotic (identical) and “the split” occurs later still, such that they not only share a placenta, but they also share an amniotic cavity and have no inter-twin membrane. These twins are at the highest risk, both because of vascular connections, and especially because of the inevitable cord entanglement. (Trevett )
Just to make things more complicated…, about 20% of monozygotic twins are formed from a single conceptus that undergoes fission in the preblastocyst stage. Such twins will actually have two placentas. Therefore, some twins that appear to be dichorionic, diamniotic on ultrasound, are actually identical genetically. We won’t know that, and clinically, zygosity is really not significant for management, but chorionicity is. As regards obstetric factors, they are at lower risk because they have a dichorionic placenta and do not share a circulation.
Monochorionicity, even without the problems of vascular anastomoses or monoamnionicity, is associated with a higher incidence of prematurity, low birth weight, weight discordance, fetal growth restriction, and poor neurological outcomes. Such pregnancies may not be best managed in the “level I” setting. It is therefore important, early in the pregnancy, to determine the chorionicity, and then decide on the best site of care. This is usually easy with early ultrasound.
