This work was designed to study the impact of prenatal knowledge of fetal sex both on the psychological and obstetrical aspects of the expectant mothers during pregnancy and labour. One hundred pregnant women attending the outpatient antenatal clinic of Assiut University Hospital were recruited. All were in the third trimester, parous, with normal pregnancy and having no past or present psychiatric disorders. The desired sex of the expected child was registered. Symptom checklist 90 (SCL-90) was applied before, and 2 weeks after sonographic fetal sex determination. Women who desired male sex scored significantly higher depressive symptoms than those who desired female sex. Women who were proven sonographically to have the undesired fetal sex showed significantly higher scores of depression, somatization, anxiety, hostility and phobia scales of SCL-90 than women whose desired fetal sex was confirmed. The second part of the study to evaluate the effect of knowing the fetal sex on the progress of labour was designed as a case control study including 57 women previously informed about their fetal sex and 40 women ignorant of their fetal sex as controls. Women delivering a baby with undesired sex showed more obstetric difficulties. In the first stage of labour, they had significant reduction in frequency of uterine contractions and rate of cervical dilatation. They also needed much more sedation, analgesia and oxytocin augmentation.