According to the American College of Obstetrics and Gynecology, overweight and tall mothers are more likely to have fraternal twins. A study shows that mothers with a BMI index of 30 or more are significantly more likely to have fraternal twins. (BMI or Body Mass Index is a measure of body fat based on a height and weight ratio.) The study also showed that taller-than-average women also have an increased chance of having twins. The mother's stature did not impact her chances of having identical or monozygotic twins, according to the study. Fraternal twinning is influenced by a number of different factors, including the use of fertility drugs, heredity and maternal age. For more information, read Determining Zygosity in Twins and Multiples and What Are the Odds?.