The responders indicated overall good adherence to the national guidelines, with the exception of early pregnancy dating based on CRL measurements. Another finding was that the management of discrepancies between methods for pregnancy dating in clinical practice varied widely, probably because of the lack of recommendations for managing such discrepancies in the national guidelines .
Although many units offered a first-trimester ultrasound examination, surprisingly few units applied pregnancy dating based on that examination. The estimated proportions of first trimester pregnancy dating were similar in comparison with 2016 register-based estimates of first-trimester (36%; 7% on CRL and 29% on BPD) and second-trimester pregnancy dating (64%) . In up to two-thirds of the units, the results of a first-trimester ultrasound examination would not be used for pregnancy dating purposes if the fetal BPD was < 21 mm. In one of 10 units, the results from any first-trimester ultrasound examination would be disregarded for pregnancy dating purposes. This is contrary to both Swedish and international guidelines, although some variation is expected, even in a small country, as is the case for other antenatal routines [4, 5, 10, 11]. However, the deviations from the guidelines were related to observed challenges after implementation, such as increased postterm rates that were attributed to the new dating formulae .
When first-trimester ultrasound results are disregarded, pregnancy dating would instead be based on second-trimester ultrasound examinations. There is a long tradition of performing mainly second-trimester ultrasound examinations in Sweden. This may explain the priority given to second-trimester pregnancy dating, which also applied when pregnancy dating had been performed in another unit. Another obstacle seemed to be the lack of reliable documentation when pregnancy dating had been performed in another county or country. Additionally, units that have not yet implemented first-trimester ultrasound examinations for chromosomal screening have less training and could be more reluctant to perform CRL-based pregnancy dating.
The predominant practice of disregarding CRL measurements, and the associated comments, indicated the units’ adherence to unwritten or informal recommendations. One reason for the use of these unwritten or informal recommendations was the observed increase in postterm rates with CRL-based pregnancy dating, which has been noted earlier, when pregnancy length is not calibrated to the same median [15, 17]. Although first trimester CRL measurements generally are more precise for pregnancy dating than second trimester measurements, the reported increase in post term rates after introducing CRL measurements for pregnancy dating could be due to problems with the used formulae or the definition of pregnancy length . A tendency to be strongly influenced by informal pathways has already been studied in the obstetric setting . However, it seemed like this practice had changed in only some counties, and almost one-third of the units still followed the written guidelines.
By contrast, adherence to specific recommendations for pregnancy dating was very high—for example in multiple or assisted reproductive therapy pregnancies. The responders made no comments indicating that the guidelines were insufficient or difficult to interpret. In general, adherence was higher than expected when compared with other studies of adherence to national guidelines [4, 5].
In many units, a follow-up would be planned if the gestational age estimated by ultrasound was 2 weeks shorter than that estimated from the last menstrual period. The vast majority also reported that in cases of a discrepancy between menstrual-period- and ultrasound-based gestational age, a new ultrasound examination would be planned after 14 days, although this is not included in the national guidelines . This may be the correct approach for assessing early growth deviations, although later growth deviations would not be addressed by this practice [13, 18, 19]. When the fetus is smaller than expected based on the last menstrual period, follow-up may be motivated by the increased risk of adverse neonatal outcomes, such as intrauterine or neonatal death [13, 18, 19]. None of the units mentioned fetal weight estimation later during pregnancy, despite the increased risk of being small for gestational age at birth in this group .
Discrepancies between the EDD by the last menstrual period and by ultrasound are common , and women’s additional information on a plausible date of conception may contradict the ultrasound-based estimated gestational age. In Sweden, clinical decisions will usually be based only on the ultrasound estimate. In some other countries, ultrasound is used for pregnancy dating only in the case of a defined discrepancy of at least 5 or 7 days between the EDD by last menstrual period and ultrasound .
We found that the units used a variety of ways to manage discrepancies between last-menstrual-period-based and ultrasound-based gestational age; this may also occur in other similar settings. Interestingly, neither national nor international guidelines mention such a discrepancy to be a risk indicator [10, 11], despite associated risks for both mother and infant, such as preeclampsia or low birthweight [13, 18, 19]. Discrepancies between methods for pregnancy dating and the suggested follow-up may need to be considered in national and international guidelines, regardless of which method is given priority for determining the EDD. Some issues to be addressed are the discrepancy threshold for follow-up and the type of follow-up that should be recommended.
The strengths of our study include the high response rate and the representation of all counties among responders: more than 84% of the pregnant population was represented in the replies. Survey studies in this setting are necessary but scarce. The study design included qualitative input by allowing free text comments, which revealed that some issues were not addressed in the national guidelines. A limitation is the study’s retrospective design, including questions on the management of pregnancy dating from 1997. Pregnancy dating in clinical practice may differ from the replies provided by the one responder who represented each unit. In addition, the nonresponders may have worked in units that differed in some aspects from those of the responders. However, the response rate was high and there were few nonresponders; therefore, we consider these results to have good generalizability.
In this evaluation of adherence to national guidelines, we identified the existence of unwritten or informal guidelines. This could be discussed during guideline revisions to ensure consensus on evidence-based guidelines to improve clinical implementation. Our findings highlight the importance of an effort to improve the implementation of guidelines in general, which ideally include a multilevel approach involving interventions between educational, practical, and policy-making areas . Local opinion leaders can have a large effect on the degree of implementation .
The observed two definitions of gestational age at EDD (39 weeks + 6 days or 40 weeks + 0 days, respectively) imply a risk when patients move between counties as 1 day of difference in gestational age could affect the induction of postterm pregnancies or differentiation of miscarriage from extremely preterm delivery . Also, using the same definition would facilitate comparisons in research .
Some units would repeat pregnancy dating performed elsewhere if the documentation was inadequate, which is in conflict with the intention to keep ultrasound exposure as low as reasonably possible to avoid adverse side effects [25, 26].
In conclusion, the units reported good adherence to national guidelines, with the exception of early pregnancy dating. The management of discrepancies between methods for pregnancy dating in clinical practice varied widely and should be considered for inclusion in the national guidelines. This study revealed that some units followed written guidelines whereas others changed practice according to unwritten informal guidelines. This indicates a need for regular updates and efforts to improve the implementation of national guidelines. Finally, follow-up of adherence to guidelines is essential and should be used as a marker of high-quality care.