Advantages of the population-based approach to pregnancy dating: results from 23,020 ultrasound examinations

Estimated date of delivery based on second trimester fetal head circumference: A population-based validation of 21 451 deliveries

INTRODUCTION

Fetal biometry is used for determining gestational age and estimated date of delivery (EDD). However, the accuracy of the EDD depends on the assumed length of pregnancy included in the calculation. This study aimed at assessing the actual pregnancy length and accuracy of EDD prediction based on fetal head circumference measured at the second trimester.

MATERIAL AND METHODS

This was a population-based observational study with the following inclusion criteria: singleton pregnancy, head circumference dating in the second trimester, spontaneous onset or induction of delivery ≥ 294 days of gestation, live birth. The EDD was set anticipating a pregnancy length of 282 days. Bias in the prediction of EDD was defined as the difference between the actual date of birth and the EDD.

RESULTS

Head circumference measurements were available for 21 451 pregnancies. Ultrasound-dated pregnancies had a median pregnancy length of 283.03 days, corresponding to a method bias of 1.03 days (95% CI; 0.89-1.16). This bias was dependent on the head circumference at dating, ranging from -1.58 days (95% CI; -3.54 to 1.12) to 3.42 days (95% CI; 1.98-4.31). The median pregnancy length, based on the last menstrual period of women with a regular menstrual cycle (n = 12 985), was 283.15 days (95% CI; 282.91-283.31). A total of 5685 (22.9%, 95% CI; 22.4% to 23.4%) and 886 women (3.6%, 95% CI; 3.3%-3.8%) were still pregnant 7 and 14 days after the EDD, respectively.

CONCLUSIONS

Second trimester head circumference measurements can be safely used to predict EDD. A revision of the pregnancy length to 283 days will reduce the bias of EDD prediction to a level comparable with other methods.

Fetal size monitoring and birth-weight prediction: a new population-based approach

OBJECTIVE

To develop a complete, population-based system for ultrasound-based fetal size monitoring and birth-weight prediction for use in the second and third trimesters of pregnancy.

METHODS

Using 31 516 ultrasound examinations from a population-based Norwegian clinical database, we constructed fetal size charts for biparietal diameter, femur length and abdominal circumference from 24 to 42 weeks' gestation. A reference curve of median birth weight for gestational age was estimated using 45 037 birth weights. We determined how individual deviations from the expected ultrasound measures predicted individual percentage deviations from expected birth weight. The predictive quality was assessed by explained variance of birth weight and receiver-operating characteristics curves for prediction of small-for-gestational age. A curve for intrauterine estimated fetal weight was constructed. Charts were smoothed using the gamlss non-linear regression method.

RESULTS

The population-based approach, using bias-free ultrasound gestational age, produces stable estimates of size-for-age and weight-for-age curves in the range 24-42 weeks' gestation. There is a close correspondence between percentage deviations and percentiles of birth weight by gestational age, making it easy to convert between the two. The variance of birth weight that can be 'explained' by ultrasound increases from 8% at 20 weeks up to 67% around term. Intrauterine estimated fetal weight is 0-106 g higher than median birth weight in the preterm period.

CONCLUSIONS

The new population-based birth-weight prediction model provides a simple summary measure, the 'percentage birth-weight deviation', to be used for fetal size monitoring throughout the third trimester. Predictive quality of the model can be measured directly from the population data. The model computes both median observed birth weight and intrauterine estimated fetal weight. Copyright © 2016 ISUOG. Published by John Wiley & Sons Ltd.

Lower accuracy in prediction of delivery date in Stockholm County following introduction of new guidelines

INTRODUCTION

In Stockholm County, new guidelines for predicting the day of delivery were introduced in 2010. Recent clinical use has indicated that the predictive quality might be suboptimal. This study compares the accuracy of three equations applied during the first (crown-rump length or bi-parietal diameter) and second trimesters (bi-parietal diameter or bi-parietal diameter combined with femur length).

MATERIAL AND METHODS

The accuracy of estimated delivery date was compared in 14 239 ultrasound exams using median deviations from actual birth date, proportion of postterms, births within ±seven days of estimated delivery date, accuracy variations depending on fetal size, alternative fetal measurements obtained on the same occasion, and menstrual age.

RESULTS

The bi-parietal diameter and crown-rump length formulae overestimated pregnancy length by two and three days respectively, causing 7-8% of pregnancies to be labelled postterm. A combined bi-parietal diameter+femur length formula overestimated by one day, with 5.1% postterms. No significant difference was found in the proportion of births within ±seven days. Second trimester estimated delivery date assessment had larger median variations than did first trimester assessment and suffered from shifting deviations across fetal size. The comparison of different biometry formulae in the same individual demonstrated one day extra deviation for bi-parietal diameter and three days extra deviation for crown-rump length compared with the combined bi-parietal diameter+femur length formula.

CONCLUSIONS

The algorithms and dating occasions tested seem inappropriate for the present 280-day term definition. Alternative formulae ought to be sought, and the assumed duration of pregnancy reconsidered; 283 days corresponds to the observed pregnancy length calculated from last menstrual period, and would better fit the observed results for first trimester ultrasound scans.

Estimation of fetal weight in pregnancies past term

INTRODUCTION

The aim of the study was to investigate the accuracy of estimating fetal weight with ultrasound in pregnancies past term, using the eSnurra algorithm.

MATERIAL AND METHODS

In all, 419 women with pregnancy length of 290 days, attending a specialist consultation at Stavanger University Hospital, Norway, were included in a prospective observational study. Fetal weight was estimated using biparietal diameter (BPD) and abdominal circumference (AC). The algorithm implemented in an electronic calculation (eSnurra) was used to compute estimated fetal weight (EFW). Results were compared with birthweight (BW).

RESULTS

The mean interval between the ultrasound examination and birth was 2 days (SD 1.4). The median difference between BW and EFW was -6 g (CI -40 to +25 g) and the median percentage error was -0.1% (95% CI -1.0 to 0.6%). The median absolute difference was 190 g (95% CI 170-207 g). The BW was within 10% of EFW in 83% (95% CI 79-87%) of cases and within 15% of EFW in 94% (95% CI 92-96%) of cases. Limits of agreement (95%) were from -553 g to +556 g. Using 5% false-positive rates, the sensitivity in detecting macrosomic and small for gestational age fetuses was 54% (95% CI 35-72%) and 49% (95% CI 35-63%), respectively.

CONCLUSION

The accuracy of fetal weight estimation was good. Clinicians should be aware of limitations related to prediction at the upper and lower end, and the importance of choosing appropriate cut-off levels.

Dating of Pregnancy in First versus Second Trimester in Relation to Post-Term Birth Rate: A Cohort Study

Objectives

To evaluate in a national standardised setting whether the performance of ultrasound dating during the first rather than the second trimester of pregnancy had consequences regarding the definition of pre- and post-term birth rates.

Methods

A cohort study of 8,551 singleton pregnancies with spontaneous delivery was performed from 2006 to 2012 at Copenhagen University Hospital, Holbæk, Denmark. We determined the duration of pregnancy calculated by last menstrual period, crown rump length (CRL), biparietal diameter (1^st trimester), BPD (2^nd trimester), and head circumference and compared mean and median durations, the mean differences, the systematic discrepancies, and the percentages of pre-term and post-term pregnancies in relation to each method. The primary outcomes were post-term and pre-term birth rates defined by different dating methods.

Results

The change from use of second to first trimester measurements for dating was associated with a significant increase in the rate of post-term deliveries from 2.1–2.9% and a significant decrease in the rate of pre-term deliveries from 5.4–4.6% caused by systematic discrepancies. Thereby 25.1% would pass 41 weeks when GA is defined by CRL and 17.3% when BPD (2^nd trimester) is used. Calibration for these discrepancies resulted in a lower post-term birth rate, from 3.1–1.4%, when first compared to second trimester dating was used.

Conclusions

Systematic discrepancies were identified when biometric formulas were used to determine duration of pregnancy. This should be corrected in clinical practice to avoid an overestimation of post-term birth and unnecessary inductions when first trimester formulas are used.

Prediction of small-for-gestational-age status by symphysis-fundus height: a registry-based population cohort study

OBJECTIVE

To develop a chart for risk of small-for-gestational-age (SGA) at birth depending on deviations in symphysis-fundus (SF) height values for gestational age during pregnancy weeks 24-42.

DESIGN

Registry-based population cohort study.

SETTING

Antenatal clinics, Västra Götaland County, Sweden, 2005-2010.

POPULATION

The study included 42 018 women with ultrasound-dated singleton pregnancies who delivered at Sahlgrenska University Hospital. Data (including 282 713 SF height measurements) were extracted from the hospital's computerised obstetric database.

METHODS

Linear and binary regression analyses were used to derive prediction models with deviations in birthweight (BW) and SF height by gestational age as dependent and independent variables, respectively. Receiver operating characteristic curves were generated to evaluate the predictive value of the model in detecting SGA.

MAIN OUTCOME MEASURES

Birthweight and small-for-gestational-age.

RESULTS

Symphysis-fundus height accounted for 3% of individual BW variance at 24 weeks, increasing gradually to 20% at 40 weeks. Maternal factors explained an additional 10 percentage points of BW variance. Receiver operating characteristic curves confirmed that SF height was a stronger SGA predictor in late than in early pregnancy. Using an SGA relative risk cut-off limit of ≥2-fold, the overall sensitivity was 50% and the overall specificity 80%. Only the most recent SF measurement was useful in predicting BW deviation; previous measurements added nothing to the predictive value.

CONCLUSIONS

The ability of SF measurements to detect SGA status at birth increases with gestational age. Only the most recent SF measurement has predictive value; a static or falling pattern of SF values did not increase SGA likelihood.

TWEETABLE ABSTRACT

New SF curves predict SGA best in late pregnancy; only the most recent SF measurement has predictive value.

Wavelet principal component analysis of fetal movement counting data preceding hospital examinations due to decreased fetal movement: a prospective cohort study

Background

Fetal movement (FM) counting is a simple and widely used method of assessing fetal well-being. However, little is known about what women perceive as decreased fetal movement (DFM) and how maternally perceived DFM is reflected in FM charts.

Methods

We analyzed FM counting data from 148 DFM events occurring in 137 pregnancies. The women counted FM daily from pregnancy week 24 until birth using a modified count-to-ten procedure. Common temporal patterns for the two weeks preceding hospital examination due to DFM were extracted from the FM charts using wavelet principal component analysis; a statistical methodology particularly developed for modeling temporal data with sudden changes, i.e. spikes that are frequently found in FM data. The association of the extracted temporal patterns with fetal complications was assessed by including the individuals’ scores on the wavelet principal components as explanatory variables in multivariable logistic regression analyses for two outcome measures: (i) complications identified during DFM-related consultations (n = 148) and (ii) fetal compromise at the time of consultation (including relevant information about birth outcome and placental pathology). The latter outcome variable was restricted to the DFM events occurring within 21 days before birth (n = 76).

Results

Analyzing the 148 and 76 DFM events, the first three main temporal FM counting patterns explained 87.2% and 87.4%, respectively, of all temporal variation in the FM charts. These three temporal patterns represented overall counting times, sudden spikes around the time of DFM events, and an inverted U-shaped pattern, explaining 75.3%, 8.6%, and 3.3% and 72.5%, 9.6%, and 5.3% of variation in the total cohort and subsample, respectively. Neither of the temporal patterns was significantly associated with the two outcome measures.

Conclusions

Acknowledging that sudden, large changes in fetal activity may be underreported in FM charts, our study showed that the temporal FM counting patterns in the two weeks preceding DFM-related consultation contributed little to identify clinically important changes in perceived FM. It thus provides insufficient information for giving detailed advice to women on when to contact health care providers. The importance of qualitative features of maternally perceived DFM should be further explored.

A new population-based reference curve for symphysis-fundus height

OBJECTIVE

To present a new gestational-age-specific percentile curve for symphysis-fundus (SF) height and to determine the effects of maternal and fetal covariates.

DESIGN

A population-based register study.

SETTING

Antenatal clinics in Västra Götaland County, Sweden, between 2005 and 2010.

POPULATION

A total of 42 018 women with ultrasound-dated singleton pregnancies who delivered at Sahlgrenska University Hospital.

MAIN OUTCOME MEASURES

Measurement of SF height.

METHODS

A non-linear regression of SF height on day of pregnancy was used to construct a reference chart for the median and other percentiles of SF height.

RESULTS

The new reference curve for SF height showed almost linear growth until term. The median value was considerably larger at each gestational age compared with the curves for SF height used in Norway and Denmark. Compared with the curve currently used in Sweden, higher median values were observed only at gestational ages >34 weeks, accompanied by an upward shift in all percentiles. The only notably influential covariates were maternal pre-pregnancy weight and height.

CONCLUSIONS

The new reference curve for SF height shows a pattern that is different from the Scandinavian reference curves of older origin, reflecting changes in the pregnant population, as well as methodological differences. The new curve can be adjusted for maternal and fetal covariates to suit individual pregnancies.