At what gestational age is an ultrasound performed to determine fetal sex?

From the Guidelines

An ultrasound to determine the sex of a fetus is typically performed between 18 and 22 weeks of pregnancy during the second trimester anatomy scan. This recommendation is based on the most recent and highest quality study, which suggests that a fetal anatomy scan is recommended at 18 to 22 weeks’ gestation to evaluate for congenital malformations 1. While this ultrasound is primarily conducted to check the baby's development and screen for anatomical abnormalities, the technician can usually identify the baby's sex at this time if the baby is positioned favorably. The sex determination is based on visualizing the external genitalia of the fetus.

Key Points to Consider

• Before 18 weeks, the genitals may not be developed enough for accurate identification, and after 22 weeks, the increasing size of the fetus and decreased amniotic fluid may make visualization more difficult.
• The accuracy of sex determination at this stage is approximately 95-100% 1.
• Some facilities offer earlier gender determination through specialized ultrasounds around 14-16 weeks, but these may be less accurate.
• Determining the baby's sex is not considered medically necessary and some healthcare providers or facilities may have policies about revealing this information.

Supporting Evidence

• A study by the Eunice Kennedy Shriver National Institute of Child Health and Human Development recommended that at least one US be offered routinely to all pregnant women between 18 and 20 weeks of gestation 1.
• The ACR-ACOG-AIUM-SMFM-SRU Practice Parameter for the Performance of Standard Diagnostic Obstetrical Ultrasound outlines the components of the standard fetal examination at 18 to 20 weeks 1.
• A recent study published in the Journal of the American College of Radiology suggests that anatomic studies performed earlier in gestation with transvaginal imaging may be helpful, especially in high-risk obese populations 1.

From the Research

Fetal Sex Determination by Ultrasound

• Fetal sex determination by ultrasound can be performed at various gestational ages, with varying degrees of accuracy 2, 3, 4, 5, 6.
• According to a study published in 1998, sonographic determination of fetal sex was attempted prospectively prior to genetic amniocentesis in 173 fetuses from 11 to 16 weeks of gestation, with a success rate of 80.3% 2.
• Another study published in 2009 found that sonographic fetal sex determination is feasible in most pregnancies, but should not be attempted before 12 weeks' gestation, as it is relatively inaccurate at this stage 3.
• After 13 weeks, the accuracy of sonographic fetal sex determination is 99% to 100% in cases without malformed external genitalia 3.

Gestational Age for Fetal Sex Determination

• A study published in 2006 found that fetal gender assignment by ultrasound at 12-14 weeks of gestation was possible in 93% of cases, with an accuracy rate of 99-100% for male gender assignment and 91.5% to 100% for female gender assignment 5.
• Another study published in 1990 found that early determination of fetal sex using transvaginal sonography was possible at 13-16 weeks of gestation, with an accuracy rate of 91.7% to 99.7% for male gender identification and 93.3% to 100% for female gender identification 6.
• A review of the literature published in 2013 found that fetal sex can be determined with a high level of accuracy by analyzing cell-free fetal DNA from 8 weeks of gestation, and by ultrasound from 13 weeks of gestation 4.

Accuracy of Fetal Sex Determination

• The accuracy of fetal sex determination by ultrasound depends on various factors, including the gestational age, the quality of the ultrasound equipment, and the experience of the sonographer 2, 3, 5, 6.
• In general, the accuracy of fetal sex determination by ultrasound increases with gestational age, with higher accuracy rates reported after 13 weeks of gestation 3, 5.