Last reviewed: October 2, 2022
Recommendation:
- Gestational age must be assessed before provision of abortion services.
- For individuals confident of the dates of their last menstrual period (LMP), gestational age may be calculated using LMP alone.
- When there is clinically relevant uncertainty about pregnancy duration using LMP alone, 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.
In practice:
-
- Bimanual examination is a routine step before intrauterine procedures and must be performed before all vacuum aspiration procedures, even when not indicated for gestational dating.
Strength of recommendation: Strong
Quality of evidence: Moderate
Importance of gestational dating
The gestational age of the pregnancy will influence the method of abortion and whether the abortion can take place at home or should take place in a facility. There are multiple ways to assess gestational age, including LMP, clinical examination of uterine size, and ultrasound. The use of LMP, alone or in combination with a validated tool such as a pregnancy dating wheel or checklist, enables individuals to self-assess gestational age (World Health Organization [WHO], 2022). Ultrasound screening for ectopic pregnancy in symptom-free individuals without risk factors is not necessary before a medical abortion (WHO, 2022); the incidence of ectopic pregnancy is lower in abortion seekers than the general population (Duncan, Reynolds-Wright, & Cameron, 2022). See 3.3: Recommendations for abortion before 13 weeks gestation: Screening for ectopic pregnancy for more information.
LMP alone
Most people can recall their LMP reasonably well regardless of their education and whether they usually record their LMP dates (Averbach et al., 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 et al., 2017; Schonberg et al., 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 LMP combined with bimanual examination 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 reported LMP alone to determine gestational age and medical abortion eligibility (Aiken et al., 2021; Reynolds-Wright et al., 2021). One study, from England, compared a cohort of 22,158 individuals who received a traditional medical abortion pre-pandemic, which included in-person assessment and routine ultrasound, to a cohort 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 people (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 participants who were up to 12 weeks gestation by self-reported LMP (Reynolds-Wright et al., 2021). Of the 663 people 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.
When asked to determine gestational age or medical abortion eligibility based on 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 et al., 2017); all three studies also evaluated participant’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%). A more recent US based study assessed the accuracy of 11 different questions for self-assessment of pregnancy duration compared to ultrasound measurements in a cohort of 1089 participants seeking abortion (Ralph et al., 2022). Using alone, 84% of participants who were ineligible for medical abortion (using a 70-day cut off) accurately identified themselves as such; when asked instead if they were more than 10 weeks pregnant, the sensitivity rose to 91%.
LMP combined with bimanual examination
Provider assessment based on 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 the accuracy of bimanual examination compared to ultrasound for gestational dating before 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 et al., 1986). A second study included 245 women and found that experienced providers using only bimanual examination 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).
Ultrasound
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 et al., 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 et al., 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 et al., 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 (American College of Obstetricians and Gynecologists & the Society of Family Planning, 2020; Kaneshiro et al., 2011; Royal College of Obstetricians and Gynaecologists, 2022; WHO, 2022).
Resources
Abortion Care Videos- Ipas: Dating a Pregancy
References
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