Introduction: The Parikh’s formula is a calculation method that considers menstrual cycle duration in women who have an unusual interval of menstruation cycle. Since the accurate estimation of gestational age affects pregnancy outcomes, the present study aimed to compare the first trimester ultrasound and Parikh’s formula in determining the Expected Date of Delivery (EDD).

Materials and methods: A prospective study was done among pregnant women referred to health centers of Borujerd, Iran, in 2014. All pregnant women with menstrual cycle less than 22 days and more than 35 days were included in the study and follow up to delivery time. Women with situations impacting the fetus development were excluded from the study. First trimester ultrasound was done at 7-13 week of last menstrual period and measured the Gestational Sac (GS) and Crown Rump Length (CRL). The gestational age was calculated by Parikh's formula. The association between the variables under study and gestational age were tested using chi-square test. The Pearson correlation coefficient was used to evaluate the correlation between the variables

Results: Overall, 300 women participated in the study. The gestational age calculated by the methods of the study, namely, the first trimester ultrasound and Parikh’s formula had no statistically significant difference (271.8 ± 0.99 and 275.2 ± 1.2 days in the first trimester ultrasound and Parikh's formula, respectively) (P=0. 625). The Pearson correlation coefficient showed a positive correlation between the GS (r = 0.645) and CRL (r = 0.768) measured by the first trimester ultrasound and gestational age calculated by the Parikh's formula.