Clinical Updates in Reproductive Health

Gestational dating

Last reviewed: January 28, 2021


  • Gestational age must be assessed before provision of abortion services.
  • For a woman confident of the dates of her last menstrual period (LMP), gestational age may be calculated using LMP alone.
  • For women with unsure LMP dates, gestational age should be assessed using estimated LMP combined with bimanual examination; ultrasound may be useful when gestational age is unclear or there is a discrepancy between the two estimates.
  • Routine use of ultrasound for gestational age determination is not necessary.

Strength of recommendation: Strong

Quality of evidence: Moderate

LMP combined with bimanual examination

Provider assessment based on women’s reported LMP combined with bimanual examination is an accurate means of determining gestational age prior to abortion (Bracken et al., 2011; Fielding, Schaff, & Nam, 2002; Kaneshiro et al., 2011). The two largest trials comparing use of LMP and bimanual examination to ultrasound prior to medical abortion up to 9 weeks gestation found that fewer than 2% of the nearly 5,000 women included would have been inappropriately offered medical abortion beyond gestational age limits if LMP and bimanual examination were relied upon to determine pregnancy duration (Bracken et al., 2011; Fielding et al., 2002).

Two small cohort studies have examined accuracy of bimanual examination compared to ultrasound for gestational dating prior to vacuum aspiration (Kulier & Kapp, 2011). In one study of 120 women, 81% of gestational age determinations made with provider assessment were concordant with ultrasound and an additional 13% were within two weeks of ultrasound estimates (Fakih, Barnea, Yarkoni, & DeCherney, 1986). A second study included 245 women and found that experienced providers using bimanual examination only to assess gestational age were within two weeks of ultrasound estimates 92% of the time, while inexperienced providers were within two weeks only 75% of the time (Nichols, Morgan, & Jensen, 2002).

LMP alone

Most women recall their LMP reasonably well regardless of their education and whether they usually record their LMP dates (Averbach, Puri, Blum, & Rocca, 2018; Harper, Ellertson & Winikoff, 2002; Wegienka & Baird, 2005). Several studies report the accuracy of LMP alone to determine gestational age compared to ultrasound prior to medical abortion (Blanchard et al., 2007; Bracken et al., 2011; Constant, Harries, Moodley, & Myer, 2017; Schonberg, Wang, Bennet, Gold, & Jackson, 2014). Two studies included a combined total of 833 women; both found that 12% of women eligible for medical abortion based on their LMP were beyond gestational age limits as determined by ultrasound dating (Blanchard et al., 2007; Constant et al., 2017). However, in the largest available study only 3.3% of 4,257 women fell into this group when a 63-day cut off value for medical abortion eligibility was used; even fewer women (1.2%) determined to be eligible by LMP were beyond 70 days gestation (Bracken et al., 2011). This study also examined the accuracy of provider assessment of pregnancy duration using both LMP and bimanual examination (see above) and found that, when this method of gestational dating was used, the rate of women who were incorrectly determined to be eligible for medical abortion decreased from 3.3% to 1.6%. A study of 660 women seeking medical abortion in Nepal compared gestational age determined by LMP to LMP plus bimanual examination without comparison to ultrasound (Averbach et al., 2018). Investigators found high agreement (99%) between the two gestational age measurements.

Two prospective cohort studies reporting on the effectiveness of telemedicine for the provision of medical abortion during the COVID-19 pandemic utilized women’s reported LMP alone to determine gestational age and medical abortion eligibility (Aiken, Lohr, Lord, Ghosh, & Starling, 2021; Reynolds-Wright, Johnstone, McCabe, Evans, & Cameron, 2021). One study, from England, compared a cohort of 22,158 women who received a traditional medical abortion pre-pandemic, which included in-person assessment and routine ultrasound, to a cohort of women who received either a telemedicine abortion (if they had a low risk of ectopic pregnancy and their self-reported LMP was consistent with a gestational age of less than 10 weeks (n=18435)) or a traditional medical abortion if they did not meet these criteria (n=11549)(Aiken et al., 2021). Treatment success, serious adverse events and incidence of ectopic pregnancy did not differ between the two cohorts; 11 women (0.04%) in the telemedicine cohort were found to have a gestational age of greater than the expected 10 weeks; all were able to complete their abortion at home without incident. A smaller Scottish cohort study followed a similar telemedicine protocol, but included women who were up to 12 weeks gestation by self-reported LMP (Reynolds-Wright et al., 2021). Of the 663 women included in the cohort, gestational age was determined using LMP alone in 79%; ultrasound was performed for uncertain gestational age in 14% and to confirm intrauterine pregnancy in 5%. Complete abortion occurred in 98% of cases, and ongoing pregnancy occurred in less than 1% of women; 2.4% of women sought additional care but no serious adverse events were reported.  

Although generally able to recall their LMP, when asked to determine gestational age or medical abortion eligibility based on that LMP, a minority of women’s assessments disagree with those of their providers.  Three studies have compared gestational age determinations made using LMP to those determined by provider assessment (Andersen et al., 2017; Ellertson et al., 2000; Shellenberg, Antobam, Griffin, Edelman, & Voetagbe, 2017); all three studies also evaluated women’s ability to self-determine their eligibility for medical abortion based on their LMP.  In the earliest of these studies (Ellertson et al., 2000), 10% of the 173 women in India who used a worksheet and their LMP to determine gestational age believed they were eligible for medical abortion, while providers determined that their pregnancies were beyond the 56-day cut off. In Nepal, 13% of 3,091 women who used their LMP and a modified gestational dating wheel to determine their medical abortion eligibility, using a 63-day cut off, were incorrect when compared to providers’ assessments (Andersen et al., 2017). Finally, in Ghana, 770 women used a modified gestational dating wheel and LMP to determine if their pregnancy was before or after 13 weeks gestation (Shellenberg et al., 2017); when compared to provider assessment, 3.6% of women incorrectly believed their pregnancies were less than 13 weeks. Of these women, one pregnancy was 13 weeks (0.1% of 770), 15 were 14 weeks (1.9%), seven were 16 weeks (0.9%), two were 18 weeks and 22 weeks (0.3% each) and one was 28 weeks (0.1%).


Ultrasound has an inherent margin of error of 3-5 days before 12 weeks gestation; this margin of error increases as the pregnancy advances (Hadlock, Shah, Kanon, & Lindsey, 1992). In studies conducted in low-resource settings–– such as India, Nepal, Vietnam and Tunisia – lack of ultrasound availability has not had an impact on the success or safety of abortion (Coyaji et al., 2001; Mundle, Elul, Anand, Kalyanwala, & Ughade, 2007; Ngoc et al., 1999; Warriner et al., 2011). Ultrasound can be helpful to establish pregnancy duration when it cannot be estimated by other methods, to confirm an intrauterine pregnancy and to identify uterine malformations (Clark, Gold, Grossman, & Winikoff, 2007; Kulier & Kapp, 2011). Dependence on routine ultrasound for gestational age determination can limit access to safe abortion services and is not necessary for accurate assessment of pregnancy duration (Kaneshiro, Edelman, Sneeringer, & Gómez Ponce de León, 2011; Royal College of Obstetricians and Gynaecologists [RCOG], 2015; World Health Organization [WHO], 2014).


Aiken, A., Lohr, P.A., Lord, J., Ghosh, N., & Starling, J. (2021). Effectiveness, safety and acceptability of no-test medical abortion provided via telemedicine: a national cohort study. BJOG: An International Journal of Obstetrics & Gynaecology. Published online 18 February 2021. Doi: 10.1111/1471-0528.16668.

Andersen, K., Fjerstad, M., Basnett, I., Neupane, S., Acre, V., Sharma, S. K., & Jackson, E. (2017). Determination of medical abortion eligibility by women and community health volunteers in Nepal: A toolkit evaluation. PLoS ONE, 12(9), e0178248.

Averbach, S., Puri, M., Blum, M., & Rocca, C. (2018). Gestational dating using last menstrual period and bimanual exam for medication abortion in pharmacies and health centers in Nepal. Contraception, 98(4), 296-300.

Blanchard, K., Cooper, D., Dickson, K., Cullingworth, L., Mavimbela, N., Von Mollendorf, C., & Winikoff, B. (2007). A comparison of women’s, providers’ and ultrasound assessments of pregnancy duration among termination of pregnancy clients in South Africa. BJOG: An International Journal of Obstetrics & Gynaecology, 114(5), 569-575.

Bracken, H., Clark, W., Lichtenberg, E., Schweikert, S., Tanenhaus, J., Barajas, A., & Winikoff, B. (2011). Alternatives to routine ultrasound for eligibility assessment prior to early termination of pregnancy with mifepristone–misoprostol. BJOG: An International Journal of Obstetrics & Gynaecology, 118(1), 17-23.

Clark, W. H., Gold, M., Grossman, D., & Winikoff, B. (2007). Can mifepristone medical abortion be simplified? A review of the evidence and questions for future research. Contraception, 75(4), 245-250.

Constant, D., Harries, J., Moodley, J., & Myer, L. (2017). Accuracy of gestational age estimation from last menstrual period among women seeking abortion in South Africa, with a view to task sharing: A mixed methods study. Reproductive Health, 14(100).

Coyaji, K., Elul, B., Krishna, U., Otiv, S., Ambardekar, S., Bopardikar, A., & Winikoff, B. (2001). Mifepristone abortion outside the urban research hospital setting in India. The Lancet, 357(9250), 120-122.

Ellertson, C., Elul, B., Ambardekar, S., Wood, L., Carroll, J., & Coyaji, K. (2000). Accuracy of assessment of pregnancy duration by women seeking early abortions. The Lancet, 355(9207), 877-881.

Fakih, M. H., Barnea, E. R., Yarkoni, S., & DeCherney, A. H. (1986). The value of real time ultrasonography in first trimester termination. Contraception, 33(6), 533-538.

Fielding, S. L., Schaff, E. A., & Nam, N. (2002). Clinicians’ perception of sonogram indication for mifepristone abortion up to 63 days. Contraception, 66(1), 27-31.

Hadlock, F., Shah, Y., Kanon, D., & Lindsey, J. (1992). Fetal crown-rump length: Reevaluation of relation to menstrual age (5-18 weeks) with high-resolution real-time US. Radiology, 182(2), 501-505.

Hamoda, H., Ashok, P. W., Flett, G. M. M., & Templeton, A. (2005). Medical abortion at 9-13 weeks’ gestation: A review of 1076 consecutive cases. Contraception, 71(5), 327-332.

Harper, C., Ellertson, C., & Winikoff, B. (2002). Could American women use mifepristone-misoprostol pills safely with less medical supervision? Contraception, 65(2), 133-142.

Kaneshiro, B., Edelman, A., Sneeringer, R. K., & Gómez Ponce de León, R. (2011). Expanding medical abortion: Can medical abortion be effectively provided without the routine use of ultrasound? Contraception, 83(3), 194-201.

Kulier, R., & Kapp, N. (2011). Comprehensive analysis of the use of pre-procedure ultrasound for first- and second-trimester abortion. Contraception, 83, 30-33.

Mundle, S., Elul, B., Anand, A., Kalyanwala, S., & Ughade, S. (2007). Increasing access to safe abortion services in rural India: Experiences with medical abortion in a primary health center. Contraception, 76, 66-70.

Nichols, M., Morgan, E., & Jensen, J. T. (2002). Ultrasound measurement for assessment of gestational age in the first trimester of pregnancy. Journal of Reproductive Medicine, 47(10), 825-828.

Ngoc, N. T. N., Winikoff, B., Clark, S., Ellertson, C., Am, K. N., Hieu, D. T., & Elul, B. (1999). Safety, efficacy and acceptability of mifepristone-misoprostol medical abortion in Vietnam. International Family Planning Perspectives, 25(1), 10-14, 33.

Reynolds-Wright, J.J., Johnstone, A., McCabe, K., Evans, E., & Cameron, S. (2021). Telemedicine medical abortion at home under 12 weeks; gestation:  A prospective observational cohort study during the COVID-19 pandemic. BMJ Sexual & Reproductive Health, Published online first 4 February 2021. Doi: 10.1136/bmjsrh-2020-200976.

Royal College of Obstetricians and Gynaecologists. (2015). Best Practice Paper No. 2: Best practice in comprehensive abortion care. London: Royal College of Obstetricians and Gynaecologists Press.

Schonberg, D., Wang, L.F., Bennett, A.H., Gold, M., & Jackson, E. (2014). The accuracy of using last menstrual period to determine gestational age for first trimester medication abortion: A systematic review. Contraception, 90(5), 480-487.

Shellenberg, K. M., Antobam, S. K, Griffin, R., Edelman, A., & Voetagbe, G. (2017). Determining the accuracy of pregnancy-length dating among women presenting for induced abortions in Ghana. International Journal of Gyncecology & Obstetrics, 139, 71-77.

Warriner, I., Wang, D., Huong, N., Thapa, K., Tamang, A., Shah, I., & Meirik, O. (2011). Can midlevel health-care providers administer early medical abortion as safely and effectively as doctors? A randomised controlled equivalence trial in Nepal. The Lancet, 377(9772), 1155-1161.

Wegienka, G., & Baird, D. D. (2005). A comparison of recalled date of last menstrual period with prospectively recorded dates. Journal of Women’s Health, 14(3), 248-252.

World Health Organization. (2014). Clinical practice handbook for safe abortion. Geneva: World Health Organization Press.