Twin pregnancies carry significantly higher risks of serious complications for both mother and babies than singleton pregnancies. More than half of all twin pregnancies deliver preterm—before 37 weeks—compared to 11.5% of singleton pregnancies, and certain conditions specific to twins can cause permanent harm or loss. The specific risks depend heavily on whether twins share a placenta. This guide explains which complications are most common, what warning signs matter, and what specialized monitoring can catch problems early when intervention works best.
Table of Contents
- How Twin Pregnancies Differ From Singleton Pregnancies
- Monochorionic Twin Pregnancies and Neurological Risk
- Twin-to-Twin Transfusion Syndrome (TTTS)
- Selective Intrauterine Growth Restriction in Twins
- Gestational Diabetes and Preeclampsia in Twin Pregnancy
- Ultrasound Monitoring Requirements for Twin Pregnancies
- Preterm Birth Risk and Prevention Strategies
- Survival and Complications When One Twin Dies In Utero
- When to Seek Specialist Fetal Medicine Care
- Aspirin Prophylaxis and Ongoing Monitoring Throughout Pregnancy
- Frequently Asked Questions
How Twin Pregnancies Differ From Singleton Pregnancies
Twin pregnancies carry multiple elevated risks from the start. More than half of all twin pregnancies deliver preterm—before 37 weeks of gestation—compared to 11.5% in singleton pregnancies, according to the American College of Obstetricians and Gynecologists. Beyond prematurity, maternal complications cluster together. Twin pregnancies carry increased risks for preeclampsia, gestational diabetes, postpartum hemorrhage, and stillbirth or neonatal death compared to singletons, requiring closer clinical monitoring and specialist fetal medicine care, ACOG states.
The shared placenta—when twins have one—introduces an entirely different layer of risk. Monochorionic twins share a single placenta whose blood vessel connections can malfunction. This creates complications unique to multiple pregnancy that singletons never face, and these conditions can develop suddenly. Twin pregnancies carry more than double the risk of gestational diabetes compared to singletons, likely due to increased placental mass and higher placental lactogen levels, according to research in the Journal of the Endocrine Society.
Gestational diabetes management becomes urgent in twins because pregnancy demands are greater. The reason for higher overall risk is biomechanical: two babies demand more from one body. Two fetuses mean double the placental tissue, double the strain on maternal organs, and in shared-placenta twins, vascular connections that can malfunction dangerously.
Your care team should identify the placentation type—dichorionic (separate placentas) or monochorionic (shared placenta)—early in pregnancy because monitoring needs differ dramatically. This distinction appears on ultrasound during the first trimester and remains the single most important classification for predicting risk.
Monochorionic Twin Pregnancies and Neurological Risk
Monochorionic twins share one placenta, which carries substantially greater risk than dichorionic twins with separate placentas. In monochorionic diamniotic twins, perinatal mortality is 2–4 times higher than in dichorionic twins or singletons, and neurological morbidity is 4–5 times higher, according to the Society for Maternal-Fetal Medicine. This higher mortality and neurological morbidity stems from the shared placenta's blood vessel connections, which can become severely imbalanced.
When blood flow becomes unequal between twins, one twin receives too much blood and the other too little. Dichorionic twins, who have separate placentas, face much lower risk of these placental complications. Dichorionic twin pregnancies have outcomes much closer to singleton pregnancies, though prematurity risk remains elevated compared to single babies. The neurological risk matters because permanent brain injury can occur silently in utero.
When one twin dies inside the womb, the surviving twin faces over 15% risk of permanent brain injury from the sudden loss of shared blood circulation. This is why monochorionic twins require more intensive monitoring than dichorionic twins. Your care team performs ultrasounds specifically designed to detect placental complications and blood flow patterns that signal danger before damage occurs.
If your ultrasound shows monochorionic placentation, expect closer surveillance starting at 16 weeks. The payoff is early detection of correctable conditions before they cause permanent damage or loss.
Twin-to-Twin Transfusion Syndrome (TTTS)
Twin-to-twin transfusion syndrome is a condition where blood flows imbalanced from one twin to the other through shared placental blood vessels. TTTS occurs in 10–15% of monochorionic pregnancies and typically appears between 15 and 26 weeks of gestation, according to the International Society of Ultrasound in Obstetrics and Gynecology. In TTTS, one twin (the donor) loses blood volume while the other (the recipient) receives too much.
The donor twin becomes smaller and produces little to no urine; the recipient twin swells with excess amniotic fluid. Left untreated, TTTS can be fatal to one or both twins. TTTS diagnosis rests on specific ultrasound findings: a size difference between twins, the donor twin having little amniotic fluid around it, and the recipient twin surrounded by excess fluid.
Your ultrasound technician is specifically trained to recognize these patterns starting at 16 weeks. Diagnosis alone is not the end of the story. Fetoscopic laser ablation of placental vessels is the recommended treatment for stage 2 or higher TTTS and can correct the blood-sharing imbalance, according to Children's Hospital of Philadelphia.
Laser treatment stops the abnormal blood vessel connections, allowing both twins to survive with normal blood flow. Laser therapy requires referral to a specialized fetal medicine center that performs the procedure. Earlier detection through biweekly ultrasound surveillance dramatically improves outcomes and survival rates. Expectant management is currently recommended only for most stage 1 cases, meaning close monitoring without immediate intervention. Stage 1 TTTS may stabilize or progress; your fetal medicine specialist guides the decision based on how your twins respond.
Selective Intrauterine Growth Restriction in Twins
Selective intrauterine growth restriction (sIUGR) is when one twin grows normally while the other becomes significantly smaller because placental blood-sharing is unequal. sIUGR affects 10–15% of monochorionic twins, according to Lurie Children's Hospital. Unlike TTTS, which shows excess fluid on the larger twin's side, sIUGR develops quietly without obvious fluid imbalance. The smaller twin falls below the 10th percentile in weight for gestational age.
Weight difference between the twins exceeds 20%, sometimes dramatically. The smaller twin's survival depends on blood flow patterns in the umbilical artery, measured on Doppler ultrasound. Type 1 sIUGR (continuous umbilical artery flow) has relatively good outcomes; Type 2 (absent/reversed diastolic flow) carries high risk of the smaller twin dying in utero; Type 3 (intermittently absent flow) carries 10–20% risk of unexpected fetal death or brain injury in either twin, according to Johns Hopkins Medicine.
Doppler patterns predict survival better than size alone. A small twin with normal continuous blood flow can survive and grow normally after delivery. A small twin with absent or reversed blood flow faces substantially higher risk of death or permanent brain injury. sIUGR requires specialist monitoring far more frequently than routine twin care. Once diagnosed, you will likely see your fetal medicine specialist every 1–2 weeks, with ultrasound Doppler assessment of blood flow guiding delivery timing.
Delivery decisions balance the risks of remaining in utero against the risks of prematurity. Your care team weighs the Doppler pattern, gestational age, and other clinical signs to determine whether delivery or continued monitoring is safer for the smaller twin.
Gestational Diabetes and Preeclampsia in Twin Pregnancy
Twin pregnancies double the risk of gestational diabetes. Twin pregnancies carry more than double the risk of gestational diabetes compared to singletons, likely due to increased placental mass and higher placental lactogen levels, according to research in the Journal of the Endocrine Society. Gestational diabetes is important to catch early in twins because it affects fetal growth and maternal health.
Screening happens around 24–28 weeks, the same as in singleton pregnancies, but your threshold for intervention is lower given the increased baseline risk. If you develop gestational diabetes, strict glycemic control may reduce the subsequent risk of preeclampsia. Preeclampsia—high blood pressure with protein in the urine developing after 20 weeks—is already more common in twin pregnancies and is a leading reason for early delivery.
Preeclampsia arrives more frequently and often earlier in twin pregnancies than in singleton pregnancies. Twin pregnancies carry increased risks for preeclampsia, gestational diabetes, postpartum hemorrhage, and stillbirth or neonatal death compared to singletons, ACOG notes. Low-dose aspirin starting in the first trimester is the standard preventive approach. The standard preventive intervention for twin pregnancy is low-dose aspirin (81 mg daily) due to the substantially increased risks of preeclampsia and placental dysfunction, according to ACOG guidelines.
Aspirin is safe in pregnancy and reduces but does not eliminate preeclampsia risk. If you develop signs of preeclampsia—severe headache, vision changes, upper abdominal pain, or unusual swelling—report them immediately to your care team.
Ultrasound Monitoring Requirements for Twin Pregnancies
Monochorionic twins require ultrasound assessment every 2 weeks starting at 16 weeks of gestation to detect TTTS and other complications; complicated pregnancies require more frequent scanning based on severity, according to ISUOG Practice Guidelines. The 16-week ultrasound serves multiple purposes in monochorionic twins. Sonographers assess fluid volume around each twin, measure their sizes, identify the number of placentas and sacs, and evaluate blood flow patterns in both umbilical cords.
After 16 weeks, monochorionic pregnancies need biweekly surveillance through the end of pregnancy. These specialized scans assess fluid volumes, the twin-to-twin weight ratio, umbilical artery Doppler patterns, and the recipient twin's heart function. A change in these measurements signals a complication requiring intervention. Dichorionic twin pregnancies require less intensive monitoring than monochorionic twins. Dichorionic twins typically receive monthly or routine prenatal ultrasounds rather than biweekly specialized scans, though your care team may increase frequency based on other risk factors.
The ultrasounds capture measurements and blood flow patterns your fetal medicine specialist uses to predict risk and time interventions. For example, if TTTS develops, ultrasound findings determine the stage and urgency of laser therapy referral. Most monochorionic twin pregnancies never develop TTTS or sIUGR, but biweekly surveillance catches problems early when treatment is most effective.
This proactive approach is why specialized monitoring improves outcomes so dramatically compared to less frequent scanning. Written ultrasound reports include specific terminology—polyhydramnios, oligohydramnios, discordance, Doppler flows—that your care team uses to communicate risk. Understanding these terms helps you recognize when your pregnancy status is changing and when escalation of care is needed.
Preterm Birth Risk and Prevention Strategies
Preterm birth affects most twin pregnancies. More than half of twins deliver before 37 weeks, and many deliver in the 34–36 week range where significant prematurity complications occur. Births before 34 weeks carry much higher risk of respiratory distress, intraventricular brain hemorrhage, and serious feeding difficulties. The reasons for preterm birth in twins are multiple: TTTS triggers urgent early delivery, sIUGR with declining Doppler flow requires early delivery for safety, preeclampsia forces delivery regardless of gestational age, and sometimes twins arrive early without an identified trigger.
Corticosteroids given to the mother before preterm delivery reduce serious complications in newborn twins. If your delivery is expected before 34 weeks, expect treatment with corticosteroids to accelerate fetal lung maturity and reduce respiratory distress and brain hemorrhage risk. Hospitalization before delivery is common when twins arrive preterm. If you develop signs of preterm labor—regular contractions, vaginal bleeding, or leaking fluid—seek evaluation immediately.
Delaying even a few days in a hospital setting allows administration of corticosteroids and antibiotics that dramatically improve newborn outcomes. Your obstetric care team can discuss your specific preterm birth risk based on placentation, any detected complications, and your medical history. Some twin pregnancies proceed safely near term; others require planned delivery by 36 weeks or earlier based on individual circumstances.
No medication reliably prevents preterm birth in twins, but reduced activity, modified work schedules, and close monitoring are standard recommendations in many cases. Your care team guides activity restrictions based on your specific situation and symptoms.
Survival and Complications When One Twin Dies In Utero
When one twin dies in utero while the other survives, the surviving twin faces serious neurological risk. A single fetal death places the surviving twin at >15% risk of permanent brain injury, according to the Society for Maternal-Fetal Medicine. The risk stems from the sudden loss of shared blood circulation in monochorionic twins.
When one twin dies, the placental connections that previously balanced blood flow now dump the dead twin's blood through the living twin, causing sudden drop in blood pressure and oxygen supply. This phenomenon is called acute twin anemia-polycythemia sequence. The surviving twin may lose a significant volume of blood to the dead twin's circulation within hours, causing severe anemia, shock, and potential brain damage if oxygen delivery is compromised.
Not all surviving twins develop brain injury after their co-twin's death. Some survive without apparent damage, but others develop seizures, cerebral palsy, or developmental delays months or years later. The outcome depends on how much blood transfer occurred and how quickly compensation happened. Early detection of fetal death through biweekly ultrasound surveillance allows your care team to plan delivery before catastrophic complications.
Delivery can often proceed vaginally, though cesarean delivery is used if delivery becomes urgent or dangerous. After delivery of twins where one was stillborn, the surviving newborn receives careful neurological assessment and often brain imaging (MRI or ultrasound) to detect injury. Some injury is apparent immediately; other damage emerges over developmental follow-up. This risk underscores why intensive monochorionic twin surveillance is so important. Biweekly ultrasounds catch fetal demise early, often before the surviving twin experiences massive, life-altering blood loss.
When to Seek Specialist Fetal Medicine Care
If you are carrying monochorionic twins, specialist fetal medicine care is considered standard of care. Your obstetrician may manage your routine prenatal care while a maternal-fetal medicine specialist performs the biweekly ultrasounds and coordinates management of any complications. Referral to a fetal medicine specialist becomes urgent if your ultrasound shows signs of TTTS—size difference between twins, fluid imbalance, or Doppler abnormalities.
A fetal medicine center with laser ablation capability should evaluate you for possible intervention. If ultrasound detects sIUGR with abnormal Doppler flow (Type 2 or Type 3), specialist consultation is essential for delivery planning. Your timing and mode of delivery depend on Doppler patterns and gestational age combined. Gestational diabetes diagnosis in a twin pregnancy warrants close coordination between obstetrics and maternal-fetal medicine.
Blood sugar targets may be tighter in twins to minimize fetal overgrowth and metabolic complications that compound prematurity risk. Preeclampsia developing before 34 weeks in a twin pregnancy requires specialist assessment of risks versus benefits of continued pregnancy. Delivery decisions in preeclampsia balance maternal safety against fetal prematurity risks unique to multiple pregnancy. Ask your obstetrician whether your pregnancy carries features that warrant specialist referral.
Monochorionic placentation alone justifies specialized ultrasound and monitoring at a center with fetal medicine expertise. If you are diagnosed with a complication during routine care, clarify whether your care team has managed that specific complication in twins before. Twin pregnancy complications require specialized knowledge different from singleton pregnancy management.
Aspirin Prophylaxis and Ongoing Monitoring Throughout Pregnancy
Low-dose aspirin (81 mg daily) is the standard preventive intervention for twin pregnancy due to the substantially increased risks of preeclampsia and placental dysfunction, according to ACOG guidelines. Aspirin is safe in pregnancy and works by reducing platelet clumping in placental blood vessels, theoretically reducing preeclampsia risk and other placental complications.
It is not a cure and does not eliminate risk, but it is evidence-based prevention. Start aspirin as soon as twin pregnancy is confirmed, ideally by 12–16 weeks gestation. Continue it daily throughout pregnancy unless your care team instructs otherwise. Aspirin at the low prevention dose does not cause miscarriage or bleeding complications. Regular blood pressure monitoring becomes more important in twin pregnancy.
If you have a home blood pressure cuff, your care team may ask you to monitor yourself between visits. Any sustained elevation above your baseline should be reported promptly. Protein in the urine is tested at every prenatal visit and serves as an early warning sign. If protein appears or blood pressure rises, your care team may diagnose preeclampsia and adjust your management or delivery timing.
Ongoing communication with your care team about new symptoms matters significantly. Twin pregnancies fatigue faster, bellies grow faster, and breathlessness is common—but sudden worsening, severe pain, vision changes, or vaginal bleeding need evaluation regardless. Your pregnancy plan should outline who to contact with urgent symptoms, which fetal movement changes matter, and what to expect at each stage. Twin pregnancies benefit from clear, written communication about your individualized plan and risk factors.
Frequently Asked Questions
What is the difference between monochorionic and dichorionic twin pregnancies?
Monochorionic twins share one placenta; dichorionic twins have separate placentas. Monochorionic twins carry 2–4 times higher perinatal mortality and 4–5 times higher neurological morbidity risk because shared placental blood vessels can become dangerously imbalanced. Dichorionic twins have outcomes much closer to singleton pregnancies. Your ultrasound can distinguish the two types early in pregnancy.
What is twin-to-twin transfusion syndrome?
TTTS occurs in 10–15% of monochorionic pregnancies when blood flows imbalanced from one twin to the other through shared placental vessels. One twin (the donor) becomes smaller with little amniotic fluid; the other (the recipient) swells with excess fluid. Laser ablation of placental vessels is the recommended treatment for advanced TTTS and can save both babies.
Can one twin die without harming the other?
No. If one twin dies in utero, the surviving twin faces over 15% risk of permanent brain injury from the sudden loss of shared blood circulation and blood transfer between the twins. Biweekly ultrasound surveillance in monochorionic pregnancies catches fetal demise early, before the surviving twin experiences dangerous blood loss.
How often should I have ultrasounds with monochorionic twins?
Monochorionic twins require ultrasound every 2 weeks starting at 16 weeks of gestation to detect TTTS, selective intrauterine growth restriction, and other complications. Complicated pregnancies may need more frequent scans. Dichorionic twins have less intensive monitoring requirements than shared-placenta twins.
Why do twin pregnancies deliver preterm so often?
More than half of twin pregnancies deliver before 37 weeks, compared to 11.5% of singletons, because of complications specific to twins (TTTS, selective intrauterine growth restriction), shared placental problems, preeclampsia, and sometimes spontaneous early labor. Biweekly surveillance catches correctable problems early.
What is selective intrauterine growth restriction?
sIUGR affects 10–15% of monochorionic twins when one twin grows normally while the other becomes significantly smaller due to unequal placental blood-sharing. The small twin's survival depends on blood flow patterns measured on Doppler ultrasound. Type 2 and Type 3 sIUGR carry high risk of fetal death or brain injury.
