The magic numbers when it comes to delivering twins may be 36 and 37, suggests a new study published Tuesday in The BMJ.

The large team of researchers reviewed data from 32 earlier studies that collectively looked at more than 35,000 twin pregnancies. They found a particular pattern for relatively healthy pregnancies where the twins had their own placenta (dichorionic pregnancies) compared to those where the twins shared the same placenta (monochorionic). The risk of death for dichorionic pregnancies was lowest when the twins were delivered around week 37; for monochorionic pregnancies, the safest time for delivery appeared to be around week 36.

“For women with dichorionic pregnancies, delivery should be considered at 37 weeks’ gestation to minimize the risk of perinatal deaths near term,” wrote the authors. “There is insufficient evidence to recommend routine delivery before 36 weeks’ gestation in monochorionic twins.”

As the authors explain, twin pregnancies are already known to be risky, especially if twins share the same placenta. In particular, monochorionic and dichorionic pregnancies may be 13 times and 5 times more likely to end in stillbirth than your typical single pregnancy, respectively. For some time now, doctors have recommended inducing twin pregnancies between weeks 34 to 37 to lower the risk of stillbirth, but that decision also comes with its share of potential harms. Simply put, the earlier a baby is delivered, the more likely they’ll suffer lingering health problems. Finding the optimal week for twin delivery has been a balancing act for doctors then, but one without any hard and fast answers.

Dichorionic twin pregnancies mostly involve fraternal twins, or twins that weren’t created from the same embryo, but 25 percent of identical twin pregnancies are also dichorionic. Because monochorionic pregnancies are rarer than dichorionic ones, though, the researchers had less information to rely on when coming up with their conclusions (their sample only looked at 5,000 or so such pregnancies). It’s estimated that 0.3 percent of all pregnancies involve twins that share a placenta but have separate amniotic sacs — even rarer are twins that share that as well.

Regardless, the researchers are optimistic that their review provides “comprehensive estimates comparing risks of stillbirth and neonatal mortality at various gestational ages, which is required for the planning of delivery in uncomplicated twin pregnancies.”

Source: Cheong-See F, Schuit E, Arroyo-Manzano D, et al. Prospective risk of stillbirth and neonatal complications in twin pregnancies: systematic review and meta-analysis. The BMJ. 2016.