Clinical estimation of fetal weight in the preterm population - an alternative to Leopold`s method validated by birth weight at the university teaching hospital, Lusaka

Abstract

Preterm birth is defined as childbirth occurring at less than 36 completed weeks or 259 days of gestation (World Health Organization, 2014). In most developing and resource limited countries such as Zambia, ultrasound estimation of fetal weight in most circumstances is not readily available for the health care professional to make decisions, a gap which can be filled by a cheaper and simpler clinical method of estimation fetal weight. One such method is the Dare’s formula which relies on the product of the symphysio-fundal height and abdominal circumference to estimate the fetal weight. The study aimed at exploring this alternative clinical method to estimate fetal weight among women at highest risk of preterm birth in a low resource population and validate it with the actual birthweight. This was a prospective study on mothers admitted to University Teaching Hospital (UTH) at risk of preterm delivery. Between 1st June and 31st October 2016, a structured questionnaire was used to collect pregnancy and outcome data on a sample size of 168 mothers that had a singleton pregnancy, longitudinal lie and known gestation <37 weeks about to deliver within one week. Maternal anthropometrics included height, weight, abdominal circumference and fundal height. The derived (estimated) fetal weight calculated using Dare’s formula was compared to the actual birthweight. Paired t-test was used to compare the mean between derived and actual weights. Multivariate analysis was used to understand what maternal or pregnancy characteristics could have led to the variance (under and over-estimate beyond 300g or 10%). Of the 168 women enrolled, over half were moderate to late preterm (32 to 37 weeks) with 134 (79.8%) were between 32-37 weeks with 54.2% between 34-<37 weeks. Very Preterm were 29 (17.3%) and only 5 (3%) were <28 weeks. Using Dare’s formula. The derived birthweight was on average 553g greater than actual birthweight (SD = 641, 95% confidence interval 456– 651, p<0.0001). There was a 71.5% chance of variance beyond 300g. On multivariate analysis for every 1cm increase in fundal height measurement the odds for above 300g weight variance reduced on average by 14% (aOR =0.86, 95% CI = 0.76 - 0.98, P= 0.0249). Similarly, for every 1cm increase in maternal abdominal circumference measurement, the odds for above 300g weight difference increased on average by 7% [adjusted Odds Ratio (aOR) = 1.07, 95% Confidence Interval (CI) = 1.03 - 1.12, P= <0.001]. Based on this study population, this clinical method and using Dare’s formula cannot be reliably used in estimation of fetal weight in preterm pregnancies. Ultrasonography remains the gold standard for determining fetal weight in preterm pregnancies and should therefore be availed as part of the tools to help in counselling mothers on perinatal prognosis. Key words: Preterm birth, Fetal weight estimation