Issues in Pregnancy Dating: Revisiting the Evidence

Linda A. Hunter, CNM, EdD


J Midwifery Womens Health. 2009;54(3):184-190. 

In This Article

Ultrasound Dating

Over the past 40 years, there has been a myriad of research studies exploring the safety, techniques, and benefits of ultrasound use in pregnancy.[20] A comprehensive review of this body of literature is beyond the scope of this article; however, there are several studies that warrant mention with regard to pregnancy dating and gestational age assessment. One of the most important of these was the Routine Antenatal Diagnostic Imaging with Ultrasound Study (RADIUS).[24] Published in 1993, this multicenter, randomized controlled trial[24] examined the efficacy of routine ultrasound screening in more than 15,000 low-risk pregnant women. Looking specifically at adverse perinatal outcomes, such as fetal death, neonatal death, or neonatal morbidity, the authors[24] found no clinical benefit to routine ultrasound screening in pregnancy. They further concluded that the costs associated with routine screening were too prohibitive to justify its use in the absence of clear medical indications.[24]

While these cost-benefit issues have been examined at length and in some cases refuted by subsequent authors,[25,26] routine ultrasound screening in pregnancy remains controversial and is not currently advocated as the standard of care in the absence of medical indications.[27,28] Regardless, sources suggest that the majority of pregnant women in the United States have at least one ultrasound during pregnancy.[5,12,20] In addition, the American College of Obstetricians and Gynecologists (ACOG)[27] states that obstetricians are under no obligation to perform routine ultrasounds in low-risk women; however, it is reasonable to honor a patient's request for one. This is an interesting sidebar that provides some latitude in an otherwise firm stance echoing the RADIUS trial findings. In spite of this ongoing debate as to the feasibility and efficacy of routine ultrasound evaluation in pregnancy, one thing remains clear: ultrasound examination of the fetus provides the most precise assessment of gestational age, especially in the first trimester.[3,6,20,27,28,29,30,31]

Moreover, as ultrasound technology has evolved, so has its accuracy in later gestation, with current guidelines reporting a consistent 8% margin of error even well into the third trimester (Table 1).[20] This variance has evolved from years of study by several esteemed researchers who have sought the most accurate measurement parameters in determining gestational age and normal patterns of fetal growth.[29,30,32,33,34,35,36]

At first glance, it may appear that some of these studies do not report consistent findings in terms of ultrasound's overall accuracy in predicting gestational age. One must keep in mind, however, that there are differences in both the statistical analysis and in how the results are reported. For example, when the margin of error is reported as a percent of the total, it is calculated against the number of days gestation verses the number of weeks. In other words, an 8% margin of error at 8 weeks' (or 56 days') gestation calculates to ± 5 days; at 18 weeks' (126 days') gestation, this represents a range of ± 10.08 days; lastly, at 28 weeks' (196 days') gestation, the margin of error continues to widen to ± 15.68 days. Regardless of these inconsistencies in reporting, most concur that an ultrasound performed before 24 weeks' gestation establishes a more accurate EDD than relying solely on the LMP.[29,30,32,33,34,35,36,37]

The implications of these findings are noteworthy when one considers the medical and legal implications of establishing a more accurate EDD. Although the RADIUS study[24] reported no significant differences in pre- or postterm delivery rates, the benefits of appropriately managing either of these clinical scenarios far outweigh the potential costs of the scan itself. Cost ratios aside, several studies have demonstrated a decrease in postterm induction rates when gestational age has been established by ultrasound.[30,31,38,39] In fact, Mongelli et al.[30] reported that postterm induction rates would be reduced by as much as 70% with early ultrasound dating in the first trimester, even with known LMP dates. Likewise Taipale and Hiilesmaa,[31] in their study of more than 17,000 women, found that the proportion of postterm deliveries decreased from 10.3% with reliable LMP dating to 2.7% when the EDD was determined by ultrasound. In more recent randomized controlled trials, researchers[38,39] have continued to report substancially fewer postterm inductions when the EDD was determined by first-trimester ultrasound.

When one also considers the costs of postterm surveillance, which typically includes twice weekly nonstress tests and weekly amniotic fluid measurements, there are potential cost savings with accurate dating by early ultrasound assessment -- not to mention the costs of fetal fibronectin cultures or the clinical and legal implications of inappropriately managing preterm labor.

Does it not then behoove all obstetric providers to discuss and offer routine ultrasound screening to every pregnant woman, even in the absence of medical indications? In midwifery practices especially, where the focus on nonintervention predominates, a single ultrasound performed before 24 weeks' gestation will not only reliably confirm the EDD but, as many of the previously cited studies suggest, may in fact reduce the likelihood of unnecessary interventions. What is most surprising, however, is that despite the evidence that supports the reliability and accuracy of ultrasound, its routine use in all pregnancies is still not recommended.[27,28] Instead, menstrual dating, with all of its potential calculation inconsistencies, recall error, and questionable origins, remains the a priori method for pregnancy dating in the absence of any complications.

Limits of Ultrasound

Ultrasound's accuracy depends greatly on the skill of the person performing the examination and the quality of the images, not to mention the size of the patient and the fetal position.[20] These technical and training issues have been addressed by the American Institute for Ultrasound in Medicine (AIUM),[28] which has set the professional standards for minimal training requirements and equipment specifications. ACOG[27] has also published specific guidelines that address training, quality assurance, safety, and clinical recommendations. Cost-benefit ratios of routine ultrasound use have still not been completely resolved in every practice setting, with one study suggesting that community-based hospitals would actually lose money if routine ultrasound screening is performed.[26] ACOG[27] further states that ultrasound sensitivity in detecting fetal anomalies remains controversial, with higher detection rates reported at tertiary centers and higher sensitivity rates overall for central nervous system and urinary tract versus cardiac anomalies.

The safety of ultrasound has also come into question, and these concerns have been addressed by the AIUM.[28] Ultrasound energy generates sound waves in a pulsed fashion that can theoretically raise the temperature of body tissues. This vibration effect, commonly referred to as cavitation, has been cited in some studies as potentially causing harm to developing fetuses.[20] Practicing what is commonly referred to as the "ALARA" principle (As Low As Reasonably Achievable), there have been no studies to date that have reported any adverse bioeffects on human fetuses as a result of exposure to the low levels of ultrasound energy in use today.[20,27,28] Furthermore, Callen's[20] critical review of these studies questioning the safety of ultrasound has shown several inconsistencies: 1) reported levels and exposure times far exceeded what is commonly in use; 2) studies done on certain plants, cell cultures, and laboratory animals are not applicable to humans; and 3) studies demonstrating adverse effects in vitro have not been reproducible in humans.

In spite of these reassurances, there remains a remote possibility that adverse effects could be identified in the future; however, current evidence indicates that the potential benefits of ultrasound far outweigh these risks.[20] Accordingly, current guidelines state that ultrasound during pregnancy should be performed only when medically indicated and at the lowest possible exposure setting.[27,28] In addition, both ACOG[40] and AIUM[41] have issued position statements discouraging the nonmedical use of ultrasound for the purposes of gender identification or keepsake videos and pictures. This position is supported by the US Food and Drug Administration, which considers the nonmedical use of ultrasound to be an unapproved use of a medical device.[42]

At the very least, careful inquiry of every woman's LMP recall and bleeding history must be taken into consideration and first-trimester ultrasound obtained if there is any question about the accuracy of this information.[10,27] If dating or viability are not the immediate issues at hand, ACOG[27] further recommends 18 to 20 weeks gestation as the optimal time to obtain a single ultrasound in pregnancy. This time frame allows for fairly accurate dating and the added benefit of a fetal survey, a factor that some authors have found significantly reduces woman's anxiety about the overall health of their baby.[2,13]

ACOG[27] also advises that providers discuss the limitations of ultrasound before obtaining any scans. Patients' wishes and feelings about ultrasound testing should be taken into consideration, and regardless of how their EDD is calculated, the rationale for this decision thoroughly explained to them and clearly documented in the prenatal record.


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