Special Feature: Contemporary Management of Twins
Special Feature
Contemporary Management of Twins
By John C. Hobbins, MD, Professor and Chief of Obstetrics, University of Colorado Health Sciences Center, Denver, is Associate Editor for OB/GYN Clinical Alert.
Dr. Hobbins reports no financial relationship to this field of study.
Prior to 1980, twins comprised about 1 in 40 pregnancies, but now, with assisted reproductive technology (ART) being widely used, the incidence of twins has risen to 1 in 20 pregnancies. It has been assumed that the increase is a result of IVF alone, but many multiple gestations occur because of the sheer number of available eggs that are released through aggressive ovarian stimulation.
Most patients, especially those who have had prolonged infertility, are quite happy about being pregnant with twins, rather than the alternative (not being pregnant). However, these patients, as well as the news media, must realize that multiple gestations are subject to higher rates of anomalies, fetal growth restriction, preterm birth, and cerebral palsy. In addition, perinatal care for twins alone represents a significant financial burden to society. One older study shows that the average hospital cost of twins delivered at 30-31 weeks was $170,000, and if born at < 29 weeks, the outlay, on average, is $500,000. This does not include the expense of long-term care of these children.
That said, with proper management, the chances of a happy ending are excellent for most twin pregnancies and, this month's Special Feature will deal with contemporary methods to maximize the outcome of twins.
Zygosity
About one-third of spontaneously conceived twins have 1 chorion and 2 amnions (mono/di configuration). The remaining two-thirds have a di/di placentation. All mono/di twins are identical and 4 of 5 di/di twins are fraternal (with 20% representing identical twins that split early, resulting in a single placenta, di/di configuration). Although one might expect that all ART twins would be di/di, actually 5%-7% is of the mono/di variety.
In virtually every category, di/di twins fare better than mono/di twins (see Table).1 Interestingly, there was a suggestion that twins from ART had better outcomes than spontaneously conceived twins, but this is only true because the ART twin pool is heavily spiked with di/di twins (96% ART vs 66% spontaneous). A recent European collaborative study involving 2-year follow-up data on mono/di twins shows a 34-fold increase in fetal death, and a 4-fold elevation in neurological impairment with ART twins vs spontaneously conceived twins.2
Since mono/di twins require more attention, it is important to assign twins a label early on, especially since it is easier to look for ultrasound signs of this type of placentation in the first trimester (membrane thickness, presence or absence of a twin peak, separate placentas, etc). Accurate gender identification comes later.
The Most Common Problems Affecting Twins
1. Anomalies. The rate of anomalies in general is about 6% (about three times higher than singletons). This increased rate also includes aneuploidy, where a patient is drawing two tickets at the lottery instead of one. For example, a mother at age 32 would have a 1 in 480 chance of having a singleton fetus with Down syndrome, but in dizygotic twins, this risk would double to 1 in 240 — about the same risk as an advanced maternal age, 35-year-old patient.
First trimester prenatal testing is particularly useful in twins because an early plan of management can be fashioned, based on a variety of findings which would include fetal size, configuration, nuchal translucency (NT) thickness, and amniotic fluid assessment. Studies have all shown that first trimester NT screening for aneuploidy and cardiac defects is particularly useful in twins.
Second trimester quad screen algorithms are adjusted for twins and are of comparable value to testing in singletons. The screening strategies that involve NT can give results for each twin (the NT being the only unique variable), but with other protocols, a single risk for either fetus having T21 is reported. A second trimester genetic sonogram requires twice the work, but the sensitivity of this test should not suffer regarding its ability to further decrease the risk for Down syndrome (by at least 50%) and to rule out major abnormalities.
2. Preterm labor (PTL). The average time when twins deliver without intervention is 37 weeks, but, as indicated above, 5% of di/di twins and 10% of mono/di twins deliver prior to 32 weeks. This can generally be predicted by cervical length (CL) exams (by transvaginal sonography) between 18-24 weeks. A group of British investigators reported that the average CL at 20-24 weeks was 3.8 cm in both singletons and twins, but in those destined to deliver prior to 32 weeks, the average CL was 2.5 cm in twins, compared with 1.5 cm in singleton pregnancies.3-5
The greatest benefit of CL is heavily weighted toward those patients whose CL is greater than 3.0 cm, since these patients can be reassured that their chances are excellent of delivering well after 32 weeks of gestation. However, what to do with patients with twins who have short cervices (< 2.5 cm) represents a dilemma, since the same British group found that cerclage is not useful in these patients, and there is one randomized trial that shows no benefit of 17 alpha-hydroxyprogesterone caproate in preventing preterm birth in twins.
3. Twin-to-twin discordance (TTD). This diagnosis has been generally applied when there is a difference in estimated fetal weight (EFW) > 20%. In di/di twins this can be due to only 3 possibilities: One twin has an anomaly or aneuploidy (the least common reason); one twin's supply line is less efficient; or, simply, one twin is genetically programmed to be smaller (the most common reason). These possibilities can be sorted out with an IUGR work-up (if the smaller twin has an EFW below the 10th percentile), which would include a search for anomalies and a Doppler waveform analysis. We have found that in most cases, perinatal morbidity is only increased in those twins whose EFW is below the 10th percentile, and/or when the discordance surfaces in the first or early second trimester. Management of IUGR twin pregnancies is predicated on exquisite timing of delivery, based on the same criteria used for singleton pregnancies, with the caveat that the decision becomes more difficult the earlier the IUGR twin shows signs of compromise, since the "innocent bystander" sharing the uterus may be in no trouble at all.
Twin-to-twin-transfusion syndrome (TTTS) occurs exclusively in mono/di twins, and, fortunately, only complicates about 10% of these twin pregnancies. However, since this condition is potentially disastrous for both twins, discordant growth in mono/di twins should put the clinician on alert. The well-known pathogenesis of this condition involves an inter-twin sharing of the placental circulation to a point where the donor twin sends a small, but essential, portion of its blood volume to the recipient twin with each heartbeat. The former twin becomes growth restricted and sometimes anemic, while the recipient is macrosomic, volume overloaded, and can be polycythemic. The current management of TTTS will be covered in a future OB/GYN Clinical Alert, but the clear current trend is to selectively ablate communicating placental vessels to interrupt what can be a lethal process for both twins.
The multicenter European study mentioned above has shed light on the meaning of TTD in mono/di twins. In 136 cases, there was a 9% incidence of TTTS.2 However, there was a TTD in 19% of those without TTTS (12% early and 7% late). This suggests that most cases of TTD in mono/di twins are not due to TTTS, and are simply due to inequities in the placental delivery systems. Perhaps the most important diagnostic clues for distinguishing the two conditions would be the early presence of an increased NT in one twin (50% of TTTS will have this), and the later presence of oligohydramnios/oligohydramnios, which occurs in virtually every case of TTTS. Ultimately, Doppler waveform analysis and fetal bladder size should separate out potentially dangerous TTTS from the less severe, or often innocuous, type of TTD.
Much of the substantial cost of twins comes from our tendency to overtest and overmanage these pregnancies. In effect, we hover over all twins for the sake of a few. The following protocol (see Figure) represents a pragmatic and frugal approach to the management of twins, which involves separating out those who need special attention, while loosening up on those who do not.
References
- Sebire NJ, et al. The hidden mortality of monochorionic twin pregnancies. Br J Obstet Gynaecol 1997;104:1203-1207.
- Ortibus E, et al. The pregnancy and long-term neurodevelopmental outcome of monochorionic diamniotic twin gestation: The multicenter prospective cohort study from the first trimester onward. Am J Obstet Gynecol 2009;200:494.e1-494.e8.
- Souka AP, et al. Cervical length at 23 weeks in twins in predicting spontaneous preterm delivery. Obstet Gynecol 1999;94:450-454.
- Heath VCF, et al. Cervical length at 23 weeks of gestation: Prediction of spontaneous preterm delivery. Ultrasound Obstet Gynecol 1998;12:312-317.
- Sperling L, et al. How to identify twins at low risk for spontaneous preterm delivery. Ultrasound Obstet Gynecol 2005;26:138-144.
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