In a study published in the journal, Fertility & Sterility, the authors performed a retrospective study using the data from the 2006-2008 SART reporting system. They reviewed 56,556 singleton and 23,804 twin live births resulting from fresh nondonor IVF cycles.
In both singleton and twin births, increased maternal height was associated with a decreased risk of preterm birth. Maternal overweight and obesity were associated with significantly increased risk of very early preterm (VEPTB; <28 weeks) and very preterm (VPTB; <32 weeks) in twin pregnancies. For very obese women (BMI > 35 kg/m) twins were associated with a threefold increased risk of VEPTB (6.1% vs. 2.0%) and a twofold increased risk of VPTB (11.5% vs. 5.9%) compared with women of normal weight (BMI 18.4-24.9 kg/m).
They concluded that obesity and short stature significantly increase the risk of VEPTB and VPTB in twins conceived by IVF. This is yet enother good reason if overweight or obese to consider getting to a more normal BMI when starting to try getting pregnant and certainly before considering IVF.