We have previously shown the critical importance of adequate pregnancy weight gain for women with twin pregnancies (https://www.mfmnyc.com/blog/mfm-publications/weight-gain-in-twin-pregnancies-and-adverse-outcomes/) and (https://www.mfmnyc.com/blog/mfm-publications/the-association-between-maternal-weight-gain-and-spontaneous-preterm-birth-in-twin-pregnancies/). However, most data has been for women who enter pregnancy with a normal weight, as defined by the pre-pregnancy body mass index (BMI). In this study, we found that women who enter pregnancy underweight should be recommended the same weight gain as women with normal pre-pregnancy weight.