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What is a Twin Pregnancy?

A twin pregnancy happens when two fetuses are growing within the uterus.

Twin pregnancies can happen for a variety of reasons, and there are several types of twins. Your chances for having a twin pregnancy increase as you age, as well as with the use of fertility therapies.

How Common Are Twin Births?

Twin births in the United States began increasing in the 1980s, but have been falling since 2014.

  • 114,161 twin births per year
  • Twin birth rate: 31.2 per 1,000 live births
  • Chance of conceiving twins: 1 out of 250 pregnancies

What Are the Different Types of Twins?

Fraternal twins (dizygotic twins):

  • Occur when two sperm fertilize two different eggs in one cycle, with a separate placenta for each baby
  • Fraternal twins may be the same or of different sex
  • Their DNA is like that of siblings of different ages

Identical twins (mono-zygotic twins):

  • Occur when a single embryo formed from one egg and one sperm divides into two early in pregnancy
  • The twins may share one or have separate placentas
  • Identical twins are always of the same sex
  • These twins are genetically identical, but may be physically different from each other

What Are the Different Types of Identical Twins?

Dichorionic, Diamniotic twins (commonly called “di-di” twins):

  • These types of twins split very early in pregnancy, they have two placentas and two double layered water sacs
  • They are at the lowest risk of complications

Monochoronic, Diamniotic twins (commonly called “mono-di” or “modi” twins):

  • These twins have one placenta and have a single layered water sac dividing them
  • MoDi fetuses are at risk for complications before birth, including Twin-Twin Transfusion syndrome (TTTS) and twin reversed arterial perfusion (TRAP) sequence
  • This type of twin requires additional assessment in pregnancy, including echocardiography and ultrasound assessments every 2 weeks starting in the second trimester
  • In rare cases, laser therapy can be needed to divide the circulation in the placenta

Monochoronic, Monoamniotic twins (“mono-mono” or “mo-mo” twins):

  • These types of twins have one placenta and are together in a single sac
  • These fetuses are at risk for complications before birth, as they can become entangled in each other’s cords
  • This type of twin requires additional assessment in pregnancy and early admission to the hospital and early delivery will be recommended

Conjoined twins:

When an identical twin embryo does not fully divide, conjoined twins can form. This means the babies’ bodies are fused, most commonly at the chest or abdomen, and may even share vital organs, like the liver or heart. Conjoined twins are extremely rare, accounting for only one out of every 100,000 twin pregnancies.

Complications in Twin Pregnancies

Twin pregnancies are at increased risk for complications for both the pregnant person and their babies. Carrying twins increases your risk for:

  • Preterm birth
  • Gestational diabetes
  • Preterm labor
  • Preeclampsia
  • Growth disturbances, like intrauterine growth restriction (IUGR)
  • Cesarean delivery

UR Medicine's Treatments for Twin Pregnancy

UR Medicine's Maternal-Fetal Care takes a patient-centered approach to all pregnancies, including twin pregnancies. We’ll make sure you see your MFM specialist early in your pregnancy to learn about your unique situation and discuss the best options for your care.

Our MFM physicians and fellows will review all the findings, and treatment options with you before any interventions. Your provider will follow the growth of your babies closely through monthly ultrasounds. They may recommend admission to the hospital, or even delivering before your due date.

Our team of MFM care coordinators are your link to your clinical team as your pregnancy progresses, ensuring you receive all the information and support you need before and during delivery at UR Medicine's Golisano Children’s Hospital.

These experienced care specialists can help with:

  • Coordinating referrals to UR Medicine’s specialists and experts
  • Navigating appointment schedules
  • Explaining complex testing and test results
  • Planning for delivery
  • Answering your questions at any point during your pregnancy

What Sets Us Apart?

UR Medicine's Maternal-Fetal Care cares for and evaluates more than 100 sets of twins each year, and is home to the largest number of perinatologists in the region. We have a method for managing every complication, including TTTS and TAPS monitoring, delivered through a care plan personalized to your needs.

Our OB/GYN MFM ultrasound unit is certified by the American Institute of Ultrasound in Medicine to perform Level II ultrasounds, a more in-depth evaluation that is needed when carrying twins.

Our experienced Registered Diagnostic Medical Sonographers and state-of-the-art technology such as 3D/4D capabilities help diagnose your developing babies effectively and accurately.

Because many twin complications include cardiac concerns, our office offers on-site fetal echocardiography. UR Medicine also offers the region’s only fetal MRI center, to help evaluate many complex conditions and placenta abnormalities. We also have fetal assessment and monitoring on-site at both of our office locations.


Our care team is here for you. Find a UR Medicine expert and get care now.

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We serve you in the Rochester metropolitan area and surrounding region.

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11 locations

Midwifery Group - Rochester

909 Culver Road, Suite 2B
Rochester, NY 14609

Reproductive Genetics - Rochester

Lattimore Medical Center
125 Lattimore Road, West Entrance, Suite 150
Rochester, NY 14620

Strong Fertility Center - Henrietta

Red Creek (Calkins Corporate Park)
500 Red Creek Drive, Suite 220
Rochester, NY 14623

Maternal-Fetal Medicine - Henrietta

500 Red Creek Drive, Suite 210
Rochester, NY 14623

Maternal-Fetal Medicine - Rochester

Lattimore Medical Center
125 Lattimore Road, West Entrance, Suite 150
Rochester, NY 14620

Maternal-Fetal Medicine - Rochester

Highland Perinatal Center
990 South Avenue, Suite 103
Rochester, NY 14620

Breastfeeding & Lactation Medicine Clinic - Rochester

Lattimore Medical Center
125 Lattimore Road, East Entrance, Suite 280
Rochester, NY 14620

Maternal-Fetal Medicine - Rochester

Highland Perinatal Center
909 Culver Road
Rochester, NY 14609

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