Twin pregnancies carry a significantly higher risk of preterm birth than singletons, with roughly 60% of twins arriving before 37 weeks. About 1 in 5 deliver before 34 weeks, and nearly 11% arrive before 32 weeks. The causes range from the sheer physical demands twins place on the uterus to pregnancy complications that occur far more frequently when two babies share the womb.
Why the Uterus Reaches Its Limits Sooner
The most fundamental cause of preterm labor in twins is uterine overdistension. Your uterus stretches significantly more to accommodate two babies, two placentas (or one shared placenta), and a larger volume of amniotic fluid. That extra stretch triggers the same hormonal and mechanical signals that normally start labor at full term, just weeks earlier. This is why even perfectly healthy twin pregnancies with no complications still tend to deliver earlier than singletons.
Cervical Shortening Patterns
The cervix acts as the gateway holding pregnancy in place, and in twin pregnancies it tends to shorten faster and earlier. Research tracking cervical length over time in twin pregnancies has identified four distinct patterns, each carrying different levels of risk.
The most concerning pattern involves early, rapid shortening starting in the early second trimester, with the cervix losing an average of about 2.3 millimeters per week. Only about 4% of women with twins show this pattern, but it’s strongly linked to very early delivery. A more common pattern, seen in roughly 25% of twin pregnancies, involves late shortening that begins around 23 to 24 weeks. Another quarter of women experience early shortening that eventually plateaus. In those cases, the level at which the cervix stops shortening matters most: each additional millimeter of cervical length at the plateau reduces preterm birth risk by about 20%.
Importantly, these patterns predict preterm birth regardless of where the cervix started. A woman whose cervix began at a normal length can still be at high risk if the rate and timing of shortening follow a concerning trajectory.
Pre-eclampsia and Medically Indicated Delivery
Not all preterm twin births happen because labor starts on its own. A large share are medically indicated, meaning doctors deliver the babies early because continuing the pregnancy would be dangerous. Pre-eclampsia is one of the leading reasons.
Twin pregnancies carry roughly 3.5 times the risk of pre-eclampsia compared to singletons. The risk of preterm pre-eclampsia specifically (the form that develops before 37 weeks and tends to be more severe) is about 9 times higher. In one large study, 5.5% to 5.8% of twin pregnancies developed preterm pre-eclampsia, compared to just 0.6% of singletons. When pre-eclampsia becomes severe, delivery is the only cure, regardless of gestational age.
Complications Unique to Identical Twins
Monochorionic twins, identical twins who share a single placenta, face an additional set of risks that fraternal twins don’t. These shared-placenta pregnancies have dramatically higher preterm birth rates: over 91% deliver before 37 weeks, compared to about 47% of fraternal (dichorionic) twins.
Twin-to-twin transfusion syndrome (TTTS) is one major driver. Because monochorionic twins share blood vessels in their placenta, blood flow can become unbalanced. One twin receives too much blood, producing excess amniotic fluid, an enlarged bladder, and potential heart failure. The other twin receives too little. The excess amniotic fluid alone can overdistend the uterus enough to trigger preterm labor, and doctors sometimes drain fluid (a procedure called amnioreduction) specifically to reduce that risk. Other shared-placenta complications like cord entanglement in monoamniotic twins and reversed blood flow conditions also frequently require early delivery.
Premature Rupture of Membranes
When the amniotic sac breaks before labor begins and before 37 weeks, it’s called preterm premature rupture of membranes (PPROM). This is a common pathway to preterm delivery in twins. The increased uterine stretch and pressure from two babies likely contributes, though research comparing twin pregnancies with and without PPROM has not identified specific risk factors that distinguish the two groups. In other words, PPROM in twins appears somewhat unpredictable, which makes it harder to prevent but important to recognize quickly.
How Preterm Risk Is Monitored
Because so many twin pregnancies deliver early, providers actively screen for warning signs. A cervical length scan offered between 16 and 20 weeks of gestation is one of the primary tools. A cervix measuring 25 millimeters or shorter flags a pregnancy as high risk.
For women already experiencing symptoms like contractions or pressure, a test that detects a specific protein in vaginal secretions can help predict whether delivery is truly imminent. In symptomatic twin pregnancies, a positive result means about a 66% chance of delivering within seven days. A negative result is even more useful: it drops that probability to just 2.4%, which can spare unnecessary hospitalization and interventions.
What Can Be Done to Delay Delivery
For twin pregnancies where the cervix has already shortened to 25 millimeters or less, daily vaginal progesterone (typically 200 mg) started between 16 and 24 weeks and continued until 34 weeks has shown meaningful benefits. It reduces the rates of very early delivery before 28 and 32 weeks, and lowers the risk of serious newborn complications. This treatment is specifically recommended for twins with a short cervix, not for all twin pregnancies across the board.
Other strategies focus on managing the complications that lead to medically indicated delivery: close monitoring for pre-eclampsia, regular ultrasounds for monochorionic twins to catch TTTS early, and amniotic fluid reduction when polyhydramnios threatens to trigger labor.
Warning Signs to Recognize
Preterm labor in twins can feel different from what many people expect. Contractions may not be dramatic or painful at first. Key symptoms to watch for include contractions every 10 minutes or more frequently, a low dull backache that doesn’t improve with position changes, menstrual-like cramping that may come with gas pains or diarrhea, and increased pelvic pressure. Changes in vaginal discharge, any leaking fluid, vaginal bleeding, and a noticeable decrease in fetal movement are all reasons to contact your provider immediately. Some women also experience nausea, vomiting, or flu-like symptoms as early labor signs.
With twins, these symptoms can start weeks earlier than expected, sometimes in the early to mid-second trimester. The threshold for calling your provider should be lower than it would be in a singleton pregnancy, because the window between first symptoms and active labor can be shorter.