Gaining weight above the Institute of Medicine recommended range contributes to the development of obesity, which increases risk for diseases. We evaluated the effectiveness of an educational program with the spouse's participation on the optimal gestational weight gain (GWG) in pregnancy. We conducted a randomized controlled trial on 128 nulliparous women attending Najmieh Hospital--Antenatal Clinic based in Tehran-Iran. Participants randomly allocated into two groups of interventions and one control group. In group A, the women received education with their spouse's participation. In group B, the women received education without the participation of the spouses. In the control group, women received only routine prenatal care. Data were analysed by the ANOVA and Chi-square coefficient. The mean of the total GWG in the groups A and B was significantly lower than the control group (13.50 3.85, 13.55 3.20 and 15.53 4.20 kg, respectively, P > 0.05). The rate of optimal GWG in groups A and B was 1.97 and 1.77 times of the control group, respectively (P < 0.05). The percentage of non-optimal GWG significantly decreased (P < 0.05). Groups A and B were not significantly different indicating no effect of spousal presence.