Preterm or not--an evaluation of estimates of gestational age in a cohort of women from Rural Papua New Guinea

Abstract

Background: Knowledge of accurate gestational age is required for comprehensive pregnancy care and is an essential component of research evaluating causes of preterm birth. In industrialised countries gestational age is determined with the help of fetal biometry in early pregnancy. Lack of ultrasound and late presentation to antenatal clinic limits this practice in low-resource settings. Instead, clinical estimators of gestational age are used, but their accuracy remains a matter of debate.

Methods: In a cohort of 688 singleton pregnancies from rural Papua New Guinea, delivery gestational age was calculated from Ballard score, last menstrual period, symphysis-pubis fundal height at first visit and quickening as well as mid- and late pregnancy fetal biometry. Published models using sequential fundal height measurements and corrected last menstrual period to estimate gestational age were also tested. Novel linear models that combined clinical measurements for gestational age estimation were developed. Predictions were compared with the reference early pregnancy ultrasound (<25 gestational weeks) using correlation, regression and Bland-Altman analyses and ranked for their capability to predict preterm birth using the harmonic mean of recall and precision (F-measure).

Results: Average bias between reference ultrasound and clinical methods ranged from 0-11 days (95% confidence levels: 14-42 days). Preterm birth was best predicted by mid-pregnancy ultrasound (F-measure: 0.72), and neuromuscular Ballard score provided the least reliable preterm birth prediction (F-measure: 0.17). The best clinical methods to predict gestational age and preterm birth were last menstrual period and fundal height (F-measures 0.35). A linear model combining both measures improved prediction of preterm birth (F-measure: 0.58).

Conclusions: Estimation of gestational age without ultrasound is prone to significant error. In the absence of ultrasound facilities, last menstrual period and fundal height are among the more reliable clinical measures. This study underlines the importance of strengthening ultrasound facilities and developing novel ways to estimate gestational age.

Fig 1. Participant Flowchart.

Fig 2. Box-and-Whisker Charts of Estimated GA at Delivery by Method.

The continuous bold line denotes the median of the reference and dashed lines denote 5% and 95% centiles of the reference.

Fig 3. Bland-Altman Plots and Levels of Agreement.

A) BS (external); B) BS (neuromuscular); C) BS (total); D) LMP; E) mid-pregnancy ultrasound; F) late-pregnancy ultrasound; G) linear SFH model; H) sequential SFH model; I) Quickening; J) corrected LMP*. The continuous horizontal lines are average levels of agreement. The dashed lines denote the 95% levels of agreement between the clinical estimators and the reference method. R represents the Pearson correlation coefficient and p values indicate significance of the parametric correlations. Significant trends are present in all comparisons, indicating significant variability in the bias across the data range. The correlations are all positive, meaning that the clinical estimators tend to further underestimate lower estimates of GA, which is demonstrated by the high number of PTB predicted by most clinical methods (Table 4).

Fig 4. Receiver Operating Characteristic Space for Mid- and Late-Pregnancy USS and Clinical Estimators to Predict PTB.

Note that the insets are magnifications of the regions of interest (outlined by the dotted lines). The solid gray lines with gray numbering are the F-measure isolines in the receiver operating characteristic space. BS(e): external BS; BS(n): neuromuscular BS; BS(t): total BS; LMP: last menstrual period; mid-scan: mid-pregnancy USS; late-scan: late-pregnancy USS; 1x SFH: linear SFH model; 3x SFH: sequential SFH model; LMP/SFH: LMP/SFH model; LMP*: corrected LMP according to PNG guidelines.