Overview Monochorionic diamniotic (Mo-Di) Monochorionic twins are twins that share a placenta and therefore a blood supply. They Diamniotic Twin occur in 3–4 per 1,000 pregnancies. Gestation They are not the type of twins that run in families. The only known risk factor is in vitro fertilization (IVF). For second opinions or About 4% of IVF pregnancies will have to speak with a health monochorionic twinning. care professional: All monochorionic twins are identical Fetal Diagnosis and twins as they share the same genetic Treatment Center material. Even though they are identical, (734) 763-4264 they do not always express the genetic uofmhealth.org/fetalcenter material the same way, so there may be some differences between the two fetuses, including the development of the anatomy and fetal growth. Executive Officers of Michigan Medicine: Marschall This material is an overview of S. Runge, M.D., Ph.D., executive vice president for monochorionic twinning and some medical afairs, dean, University of Michigan Medical School, CEO, Michigan Medicine; David A. Spahlinger, general management recommendations. M.D., president, UMHS, and executive vice dean for As each woman’s pregnancy is different, clinical afairs, University of Michigan Medical School; we suggest seeking out additional Patricia D. Hurn, Ph.D., dean, School of Nursing. information and care plans from your Regents of the University of Michigan: Jordan B. obstetric provider or a Maternal Fetal Acker, Michael J. Behm, Mark J. Bernstein, Paul W. Brown, Shauna Ryder Diggs, Denise Ilitch, Ron Weiser, Medicine physician to understand issues Katherine E. White, Mark S. Schlissel, ex officio. that may be unique to you. A Non-discriminatory, Affirmative Action Employer. © 2020, The Regents of the University of Michigan” Monochorionic Diamniotic What Are the Increased Monitoring Twin Gestations Risks With Monochorionic Diamniotic Twins? • Monochorionic diamniotic twins Due to the unique complications are identical twins who share • A higher risk of birth defects makes associated with monochorionic a placenta (monochorionic) thorough ultrasound evaluation of the fetal twins, management includes: but each have their own inner anatomy very important. sac (diamniotic). This type of • Early scan at <14 weeks to twinning (or twin pregnancy) • Intrauterine growth restriction (IUGR) confirm chorionicity accounts for approximately 20% means the fetus is smaller than expected. of all twins. Selective IUGR (sIUGR) indicates only one • Ultrasounds every 2 weeks fetus is affected, and this occurs in 12–25% beginning at 16 weeks to • Even though most of monochorionic twins. This is most often evaluate for complications monochorionic twins are caused by unequal placental sharing or healthy, they have a much abnormal umbilical cord insertions, such as • Increasing frequency of higher risk of complications velamentous cord insertion into the amniotic ultrasound if either fetus has due to a shared blood supply. membranes instead of the placenta. increased or decreased amniotic Because of this risk, it is fluid or abnormal growth or important to monitor them • Twin-to-twin transfusion syndrome (TTTS) Doppler blood flow closely. occurs in 10–15% of monochorionic twins due to abnormal placental blood vessel • Twice weekly antenatal testing • The most important first step is connections. TTTS is caused by one twin at 32 weeks gestation, if not checking to see if twins share (donor) sending excess blood to the other started earlier the placenta. This is best done twin (recipient). Early stages are detected by with an ultrasound before 14 ultrasound when the donor has low amniotic • Delivery at 37 weeks gestation, weeks gestation. fluid (oligohydramnios) and recipient has or earlier if indicated too much amniotic fluid (polyhydramnios). If • If there is confirmation untreated, it is life-threatening for both twins. of monochorionicity, it is recommended to start close • Twin anemia polycythemia syndrome ultrasound monitoring at 16 (TAPS) is rare. In this condition, one fetus weeks gestation. (donor) develops anemia (low blood count) and the other (recipient) has an abnormally high blood count (polycythemia)..