Abstract: Background: Parents feel strongly about whether or not to learn the sex of their fetus. We sought to determine which factors are significantly associated with parents’ desire to know or not to know the fetal sex during a prenatal ultrasound. Methods: All women undergoing prenatal ultrasound examinations, except for those with suspected failed pregnancies, were invited to answer a questionnaire at an outpatient referral center for diagnostic ultrasound in obstetrics and gynecology in Boston, Massachusetts. The survey asked about demographic factors, current pregnancy, and past pregnancies, and an open-ended question about whether and why the parents wished to learn, or did not learn, the sex of their fetus. Factors significantly associated with parents’ desire to learn the fetal sex prenatally were determined and analyzed. Results: A total of 1,340 questionnaires were completed. Overall, 761/1,302 (58%) of mothers and 747/1,295 (58%) of fathers learned or planned to learn the fetal sex before delivery. Factors most associated with wanting to learn the fetal sex were conceiving accidentally, finding out the sex in a previous pregnancy, not planning to breastfeed, influence of sex on future childbearing plans, planning a move or renovation dependent on sex, and specific parental sex preference. Demographic factors most associated with wanting to learn the fetal sex were father without full-time job, lower household income, unwed mother, maternal age less than 22 or greater than 40 years, no college degree, race other than white, and religion other than Catholic. Conclusions: Specific demographic and socioeconomic characteristics predicted whether or not parents chose to know the sex of their unborn child. Families in which the pregnancy was unplanned, those in which fetal sex would influence living arrangements or future childbearing plans, and those of lower socioeconomic status wished to know the sex more frequently. Further study is needed to understand parents’ motivations underlying the desire to know or not know fetal sex before delivery.