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Management of Multi-Fetal Pregnancy: monochorionic, diamnionic twins; monochorionic, monoamnionic twins

Nov 2012 – by J. Repke, MD

Twin pregnancy now accounts for about 3% of all live births. Di-zygotic twins (dichorionic) represent about 70% of all twins with the remaining 30% being monozygotic (dichorionic or monochorionic). The incidence of monozygotic twins worldwide is about 1 in 250. The incidence of di-zygotic twins is about 1 in 30, an increase over the past 20 years largely due to both the increased use of artificial reproductive technologies and the increasing maternal age of the pregnancy population.

Diagnosis: Ideally twins should be diagnosed as early in gestation as possible. The most reliable method of diagnosis is ultrasound. In the first trimester either a thickened septum between the chorionic sacs, or the presence of the lambda or “twin-peak” sign which represents the same thickening of the chorion at the base of the inter-twin membrane is the best method of determining a dichorionic gestation. This identification is important because dichorionic gestations tend to be at lower risk for complications and, provided that fetal growth remains concordant, can generally be managed with relative ease. Close surveillance for medical complications of pregnancy such as premature labor, preeclampsia and gestational diabetes is warranted in all twins since the rates of these disorders are higher.

Monochorionic - Diamnionic Twins (mono-di) twins have the potential for more complications than dichorionic twins. This is largely related to the potential for shared circulation. This circulatory anomaly, when it occurs, can lead to the so-called twin-twin transfusion syndrome (TTTS) (also called polyhydramnios-oligohydramnios sequence) that can result in significant growth discordance, and potentially fetal loss. It is recommended that surveillance of mono-di twins begin with an ultrasound at 16 weeks, when the earliest signs of TTTS may appear, and that serial ultrasonography for assessment of fetal growth and well-being be performed at 3-4 week intervals and more frequently if there are findings of concern. TTTS prognosis is largely dependent on gestational age at diagnosis (the earlier it occurs the worse the prognosis) and severity of the discordance. Treatment is based on a staging system of findings and both laser therapy and serial amniocentesis have been used to try to manage and treat this disorder, but current literature would favor the laser approach, the goal being to obliterate those shared circulation vessels and restore normal placental–fetal hemodynamics. Because of the potential for shared vessels, it is recommended that immediate delivery be considered if a viable gestational age has been reached and there appears to be a risk of imminent demise of one twin. If one fetus does die in utero, early delivery of the surviving twin is not warranted (provided that the surviving twin is showing no signs of compromise), because the potential for damage (resulting from profound hypotension and cerebral ischemia with resulting porencephalic cyst formation and periventricular leucomalacia) has already occurred.

In the absence of discordance or other complications, routine obstetrical management is sufficient, with ultrasound examinations every 3 to 4 weeks as outlined above. The role for antenatal testing in uncomplicated mono-di twins has not been established and there are insufficient data to either endorse or dispute this practice. Vaginal delivery can occur with the onset of spontaneous labor if the presenting fetus is vertex, there are no contraindications to vaginal delivery, and if the non-presenting twin is not substantially larger than the presenting twin. If the non-presenting twin is other than vertex, external version may be attempted after delivery of the first twin. In general, if the provider is not experienced in breech extraction it is acceptable to offer primary cesarean section to the patient as a vaginal delivery of twin “A” with a cesarean delivery needed for twin “B” has higher maternal and fetal morbidity associated with it than either a successful vaginal delivery of both twins or a cesarean delivery for both twins.

Monochorionic –Monoamnionic (mono-mono) Twins is fortunately a rare condition (about 3% of monochorionic twin gestations) and perinatal mortality can be in excess of 20% with cord entanglement being the leading cause of fetal death. The optimal management of mono-mono twins is unclear and the condition is too rare to allow for meaningful data from clinical trials to exist. Many advocate for hospitalization in the 3rd trimester with intensive fetal surveillance. Due to the impracticality of continuous electronic fetal heart rate monitoring for weeks on end, cesarean delivery at 32 weeks has been recommended by many in order to avoid unexpected fetal demise.


1. ACOG Practice Bulletin. Multiple Gestation: Complicated Twin, Triplet and High-Order Multi-Fetal Pregnancy. #56 October 2004 (Re-affirmed 2010).
2. Ramsay PS, Repke JT. Intrapartum Management of Multi-Fetal Pregnancies. Semin Perinatol. 2003;27:54-72
3. Baxi LV, Walsh CA. Monoamniotic twins in contemporary practice: a single-center study of perinatal outcomes. J Matern Fetal Neonatal Med 2010;23:506