AN ADJUSTABLE FETAL WEIGHT STANDARD

The monitoring of fetal weight is an important aspect of antenatal car e. To construct an individually adjustable standard, we developed a mo del to link the predicted birth weight to a fetal weight curve which o utlines how this weight is to be reached in an uncomplicated pregnancy . A formula was derived which describes the median fetal weight at eac h gestation as a proportion of the optimal term weight, and also defin es the 90th and 10th centile curves as normal limits. We analyzed a bi rth weight database of 38 114 singleton, routine ultrasound-dated preg nancies resulting in term deliveries. By stepwise multiple regression analysis, we derived coefficients for the factors that act as variable s on term birth weight in our population. Apart from gestational age a nd sex, the maternal height, weight at first visit, ethnic group, pari ty and smoking all have significant and independent effects on birth w eight. The variation due to ethnic group appears to be physiological i n this population. Smoking is associated with a reduction in birth wei ght, which is independent of maternal physique and related to the numb er of cigarettes per day as reported at the first visit.We have develo ped a software program which calculates, on the basis of pregnancy var iables entered at the first visit, an adjusted normal range for fetal size. This can be printed out as a chart and used for antenatal survei llance of growth.