skip to Cookie NoticeSkip to contents

The health and safety of our patients, visitors and staff remains our top priority. Learn more about our Safe Care Commitment. For faster entry, use our Prescreen app prior to arrival.

Header Skipped.

Pregnancy with Twins, Triplets and More

Expecting twins, triplets or more can be both exciting and overwhelming. Many aspects of a twin multiple pregnancy are different than a singleton pregnancy. If your obstetrician has told you that you’re carrying twins or more, you probably have many questions: How can I stay healthy during my pregnancy? What should I do to keep my babies healthy? Which complications are possible during a twin pregnancy and delivery? What additional testing do I need?

The maternal-fetal medicine specialists at Brigham and Women’s Hospital (BWH) have provided this information as a guideline and a supplement to what your own obstetrician has or reproductive specialist may already have discussed with you.

Printable Illustrated Diagram of the Types of Twins

Twin Pregnancy Overview

Types of Twins

The care and monitoring of your twin pregnancy depends upon the type of twins you are carrying. Your obstetrician will determine this at your first-trimester ultrasound.

Fraternal (Dizygotic Twins)

  • These twins are conceived when two separate eggs are fertilized by two different sperm. The two separate embryos are implanted in the uterus at the same time.
  • They are essentially siblings born at the same time. They have different genetic makeups and may or may not be the same gender.

Identical (Monozygotic Twins)

  • These twins are conceived when one egg becomes fertilized and then splits into two separate embryos.
  • They will be the same gender and have nearly the same genetic makeup, but may not be exactly identical because some genes will be expressed differently.
Classification of Identical Twins

Your doctor will identify your twins based on the placenta and amniotic sac.

Dichorionic Diamniotic (Di-Di) Twins

  • Separation into two embryos occurs 1-3 days after fertilization
  • Each twin has his or her own placenta and amniotic sac

Monochorionic Diamniotic (Mo-Di) Twins

  • Separation into two embryos occurs 4-8 days after fertilization
  • Each twin has his or her own amnion and amniotic sac
  • The twins share a placenta but each has his or her own amniotic sacs

Monochorionic Monoamniotic (Mo-Mo) Twins

  • Separation occurs 8-13 days after fertilization
  • The twins share the same placenta and amniotic sac

Conjoined twins

  • Separation occurs 13-15 days after fertilization
  • The twins share the same placenta and amniotic sac and they share some organs

Click here for a printable illustrated diagram of the types of twins.

Recommended Office Visits

Below you will find the recommended timing of office visits for uncomplicated twin pregnancies. If your pregnancy is affected by a maternal or fetal complication, you should expect to have more frequent office visits.

Twin Pregnancy Recommended Protocols




Dichorionic-Diamniotic (Di-Di)

  • First prenatal visit
  • 28 weeks
  • 36 weeks
  • Every four weeks
  • Every two weeks
  • Every week until delivery

Monochorionic-Diamniotic (Mo-Di)

  • First prenatal visit
  • 16 weeks
  • 36 weeks
  • Every four weeks
  • Every two weeks
  • Every week until delivery

Monochorionic-Monoamniotic (Mo-Mo)

  • First prenatal visit
  • 16 weeks
  • Recommend inpatient admission with daily fetal surveillance at 24-28 weeks (after individualized discussion with patient)
  • Every four weeks
  • Every two weeks

A twin pregnancy requires more frequent visits to the obstetrician and ultrasounds to monitor the mother’s health, the twins’ growth rates, and any signs of complications. Below are our guidelines for ultrasounds throughout your twin pregnancy, but keep in mind that your twins may need more frequent ultrasound surveillance.




Determine the date of the pregnancy and the type of twins

First trimester

  • Optimal during 7-10 weeks
  • Measure twins (often referred to as the “crown-rump length”)

Aneuploidy screening

10-13 weeks of gestation

  • Assess each twin’s individual risk of having an aneuploidy malformation and/or twin-twin transfusion syndrome

Scan each twin’s anatomy

Second trimester

  • Optimal during 18-20 weeks
  • Assess for structural abnormalities

Follow up for Di-Di twins

Starting at week 24

  • Assess every 4 weeks for uncomplicated Di-Di twins

Follow up for Mo-Di and Mo-Mo twins

Starting at week 16

  • Assess bladder and amniotic fluid every 2 weeks
  • Assess growth every 4 weeks for Mo-Di twins
  • Assess more frequently for Mo-Mo twins

Adapted from: Reddy, UM, Abuhamad AZ, Levine D, Saade GR for the Fetal Imaging Workshop Invited Participants. Fetal imaging: Executive Summary of a Joint Eunice Kennedy Shriver National Institute of Child Health and Human Development, Society for Maternal-Fetal Medicine, American Institute of Ultrasound in Medicine, American College of Obstetricians and Gynecologists, American College of Radiology, Society for Pediatric Radiology, and Society of Radiologists in Ultrasound Fetal Imaging Workshop. Am J Obstet Gynecol 2014; 210 (5): 387-397.

Prenatal Diagnosis

Aneuploidy Screening

  • A woman carrying twins—regardless of age—should receive routine aneuploidy screening. There are a number of screening tests available and the different options should be discussed with your obstetric care provider.
  • Invasive diagnostic testing with chorionic villus sampling (CVS) or amniocentesis are also available options.

Structural Anomalies

  • A dizygotic twin pregnancy (generally dichorionic) has a similar rate of congenital anomalies as a singleton pregnancy. However, a monozygotic twin pregnancy (generally monochorionic) has a higher risk for congenital anomalies in one or both twins.
  • During the second trimester, the anatomy ultrasound will screen for possible abnormalities.
  • If one twin is diagnosed with an anomaly, this can be challenging because decisions regarding monitoring, therapy and delivery can affect both fetuses.
  • If a fetal anomaly is detected, consult with an experienced maternal-fetal medicine specialist to guide you and help you make decisions.

While pregnant with twins, you will have unique nutritional needs, especially for additional calories, vitamins and minerals. You may, however, experience loss of appetite or feelings of fullness. To help ensure that you’re eating a healthy diet for you and your babies, you should consider having a nutrition consultation during your pregnancy.

During your twin pregnancy, you can expect to:

  • Increase you daily dietary intake by about 600 calories per day, 300 calories more than a woman carrying one baby. High-protein shakes are a good choice because of the high nutritional value.
  • Monitor your vitamin and mineral intake closely. The Society of Maternal-Fetal Medicine recommends that you take 30 milligrams of iron during the first trimester and 60 milligrams until delivery to prevent anemia. The Society of Maternal-Fetal Medicine also recommends that women take 1 mg of folic acid to prevent neural tube defects (most prenatal vitamins have 0.4 to 0.8 mg of folic acid so extra supplementation is recommended).
  • While carrying twins, you will need to gain more weight than for a single pregnancy. Below are the Institute of Medicine’s recommendations for weight gain.



<18.5 (underweight)

No recommendation due to insufficient data.

18.5-24.9 (normal weight)

37-54 pounds or 16.8-24.5 kilograms

25.0-29.9 (overweight)

31-50 pounds or 14.1-22.7 kilograms

≥30.0 (obese)

25-42 pounds or 11.4-19.1 kilograms

When to Consult a Maternal-Fetal Medicine Specialist?

In addition to your routine care from your obstetrician, you may want to consult with a maternal-fetal medicine specialist to receive specialized care. This is especially recommended if you are carrying:

  • Di-Di twins with complications such as discordant growth, high risk of preterm delivery or other fetal anomalies.
  • Mo-Di twins with concern for fetal anomalies, unequal placental sharing, discordant growth or twin-to-twin transfusion syndrome (TTTS).
  • Mo-Mo twins require closer surveillance. Brigham and Women’s Hospital recommends transfer to a maternal-fetal medicine specialist for inpatient admission between 24-28 weeks.

Have a Question?

We are happy to respond to general questions about twin pregnancies. Please use our online form to send us your twin pregnancy question, and one of our maternal-fetal medicine specialists will respond. Please note that your question will be emailed and responded through a private and secure system.

Multiple Pregnancy Program Leaders

Julian N. Robinson, MD
Division of Maternal-Fetal Medicine

Carolina Bibbo, MD
Division of Maternal-Fetal Medicine

Words You Should Know

Amniocentesis: A test to identify certain genetic conditions using a sample of the amniotic fluid. It is usually done after 15 weeks.

Amniotic Sac: The sac that contains the fetus.

Aneuploidy: A condition in which there is an extra or missing chromosome.

Body Mass Index (BMI): A measure of a person’s weight in relation to height.

Cesarean Delivery: Delivery of a baby through an incision made in the mother’s abdomen and uterus.

Chorionic Sac: The outer sac that surrounds and protects the amniotic sac and embryo.

Chorionic Villus Sampling (CVS): A test to identify certain genetic conditions using sample cells from the placenta. It is usually performed from 10-13 weeks.

Cord Entanglement: A complication among Mo-Mo twins in which their umbilical cords become entangled.

Embryo: The fertilized egg from conception to about 8 weeks, when it is then called a “fetus.”

Fetus: The developing, unborn baby.

Fraternal Twins (Dizygotic Twins): Twins that are conceived when two separate eggs are fertilized and implanted in the uterus at the same time; they have different genetic makeup.

Gestation: The period between the conception and birth of the child.

Hypertension: High blood pressure.

Identical Twins (Monozygotic Twins): Twins that are conceived when one egg becomes fertilized and then splits into two separate embryos; they share the same genetic makeup.

Intrauterine Growth Restriction (IUGR): A condition in which a fetus grows more slowly than the normal rate.

Placenta: The organ that connects the developing fetus to the uterus to deliver the baby nutrients and oxygen, eliminate waste, and produce hormones to support the pregnancy.

Preeclampsia: A complex disorder in which a pregnant woman has high blood pressure and often a high protein count in her urine after 20 weeks of gestation.

Prenatal: During the pregnancy, before the birth of the child.

Preterm Delivery: Delivery before 37 weeks of gestation.

Sciatica: Pain that exudes from the sciatic nerve, typically on one side of the body’s lower back through the hip, buttock, and leg.

Singleton Pregnancy: The pregnancy of a single child.

Structural Anomaly: When the baby’s body part is altered in some way.

Trimester: One third of the pregnancy duration; the first trimester is week 1 through 13, the second trimester is week 13 through 26, and the third trimester is week 26 through the birth of the child.

Twin Reversal Arterial Perfusion Syndrome (TRAP): A rare condition of Mo-Di twin pregnancy in which one twin does not develop a heart and brain structures but receives blood supply from the other, normally developing twin.

Twin-to-Twin Transfusion Syndrome (TTTS): A complication of Mo-Di twin pregnancy in which the fetuses’ vascular connections become intertwined in their shared placenta.

Ultrasound: A type of technology that produces images of a fetus within the womb and is used for monitoring the fetus’ development.

Vertex Position/Presentation: When the baby’s head is down, pointing directly toward the birth canal.

Zygosity: The condition of being developed from one or more eggs.

Zygote: The fertilized egg cell that results from the union of a female gamete (egg or ovum) with a male gamete (sperm).


Learn about obstetric services provided by Brigham and Women’s Hospital.

Read more about maternal-fetal medicine at Brigham and Women’s Hospital.

Common questions and answers for women expecting twins from the American College of Obstetrics and Gynecology.

Social support organizations offer helpful information:


For over a century, a leader in patient care, medical education and research, with expertise in virtually every specialty of medicine and surgery.

About BWH