Clinical Updates in Reproductive Health

Gestational dating

Last reviewed: January 6, 2020


  • Gestational age should be calculated using a woman’s last menstrual period (LMP) combined with physical examination.
  • Routine use of ultrasound for gestational age determination is not necessary.

Strength of recommendation: Strong

Quality of evidence: Very low

Importance of accurate gestational dating

Errors in gestational dating can increase the risks associated with abortion. If gestational age is underestimated prior to dilatation and evacuation (D&E), providers may not have the experience and equipment to complete the procedure safely. Accurate assessment of gestational age enables providers to determine whether the facility is equipped to provide the requested service and refer to another facility if necessary.


Gestational age assessment using bimanual examination and women’s LMP is well established during prenatal care, as is the use of ultrasound. No trials have compared the accuracy of different methods of gestational dating prior to abortion at or after 13 weeks, and a 2011 systematic review aimed at determining if preabortion ultrasound affected the safety or efficacy of abortion procedures both before and after 13 weeks found no relevant studies (Kulier & Kapp, 2011). In the United States, virtually all providers use ultrasound for gestational age assessment after 12 weeks gestation, but data are lacking from other country contexts (O’Connell et al., 2018).

Prior to medical abortion, gestational age can be estimated using the first day of a woman’s LMP and a physical examination that includes bimanual and abdominal examination (Nautiyal, Mukherjee, Perhar, & Banerjee, 2015; Ngoc et al., 2011; Royal College of Obstetricians and Gynaecologists [RCOG], 2015; World Health Organization [WHO], 2014). Measuring fundal height, as in routine obstetric care, can provide additional information as the pregnancy advances (Pugh et al., 2018). Ultrasound can be used to confirm gestational age if the LMP and clinical examination are discordant or if there is uncertainty about gestational age but is not required prior to medical abortion.

In published studies of D&E, including reports of implementation of D&E programs (Castleman, Oanh, Hyman, Thuy, & Blumenthal, 2006; Jacot et al., 1993), ultrasound has been routinely used to establish or confirm gestational age prior to D&E. However, one published report (Altman, Stubblefield, Schlam, Loberfeld, & Osanthanondh, 1985), unpublished programmatic data (A. Edelman, personal communication, January 12, 2018) and expert opinion support use of LMP and physical examination for gestational age assessment, with use of ultrasound as needed (RCOG, 2015; WHO, 2014). If ultrasound is used, biparietal diameter is a simple and accurate method to confirm gestational age (Goldstein & Reeves, 2009). A femur length measurement can be used to confirm the biparietal diameter or used if there are technical difficulties in obtaining a biparietal measurement.

Women who present with fetal demise, incomplete abortion or for postabortion care may have discordant LMP dates and uterine size; they should be treated according to uterine size (RCOG, 2016; WHO, 2018).

After the abortion, clinicians can confirm gestational age by comparing actual fetal measurements (fetal foot length) to the expected gestational age (Drey, Kang, McFarland, & Darney, 2005; Mokkarala, Creinin, Wilson, Yee, & Hou, 2020). This comparison provides feedback regarding the accuracy of pre-procedure dating estimates. Pregnancy dating tools, such as fetal measurements, are included in Ipas’s Dilatation & Evacuation (D&E) Reference Guide: Induced Abortion and Postabortion Care at or After 13 Weeks Gestation, page 38 (2017), and Medical Abortion Reference Guide: Induced Abortion and Postabortion Care at or After 13 Weeks Gestation, page 30 (2017).

Identification of fetal sex with ultrasound

After 14 weeks gestation ultrasonographic visualization of the male penis or female labial folds can be used to determine fetal sex in approximately 90% of gestations (Gelaw & Bisrat, 2011; Meagher & Davidson, 1996; Watson, 1999; Whitlow, Lazanakis & Economides, 1999).

Before 14 weeks, male and female genitals are similar in size and appearance on ultrasound (Feldman & Smith, 1975) and sex determination must instead be made by identification and evaluation of the genital tubercle (a protuberance on the lower ventral wall of the embryo that eventually becomes the penis or clitoris), (Efrat, Akinfewa, & Nicolaides, 1999). A 2013 review of studies determining fetal sex through ultrasonographic evaluation of the genital tubercle (Colmant, Morin-Surroca, Fuchs, Fernandez, & Senat, 2013) found that sex determination via this method was unreliable below 12 weeks gestation. Findings published since the review have been similar (Gonzalez Ballano, Saviron Cornudella, Puertas, & Luis, 2015; Lubusky, Studnickova, Skrivanek, Vomackova, & Prochazka, 2012; Manzanares, Benitez, Naveiro-Fuentes, Lopez-Criado & Sanchez-Gila, 2016).

Regardless of the method employed to determine fetal sex, accuracy improves with increasing gestational age (Elejalde, Elejalde, & Heitman, 1985; Colmant et al., 2013) and skill of the ultrasonographer (Lubusky et al., 2012). Unfavorable fetal position and a woman’s body habitus may limit the ability to determine fetal sex regardless of gestational age or ultrasonographer skill (Behrendt, Foy, Center, & Durnwald, 2012; Efrat, Perri, Ramati, Tugendreich, & Meizner, 2006; Elejalde et al., 1985).


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