Estimated Fetal Weight (EFW) inpregnancy

    It is not possible to weigh the fetus in utero. It is therefore by biometry, that is to say the measurement of the fetus by ultrasound, that we can have an estimate of the fetal weight. This is done during the second ultrasound (around 22 weeks) and the third ultrasound (around 32 weeks).

    The practitioner will measure different parts of the fetus' body:

    This biometric data, expressed in millimeters, is then entered into a mathematical formula to obtain an estimate of the fetal weight in grams. The fetal ultrasound machine performs this calculation. The result is indicated on the ultrasound report with the words "EPF", for "Estimated fetal weight".

    There are different curves around the world but currently it is recommended to use the WHO EPF curve, because this reference reports a proportion of fetuses screened as SGA (small for gestational age) and LGA (large for gestational age) adapted to the French population and good performance in identifying fetuses at risk.

    The result obtained, however, remains an estimate. Most formulas have been validated for birth weights of 2,500 to 4,000 g, with a margin of error compared to the actual birth weight with a median of 5.7% (which corresponded to 200 g), due in part to the quality and precision of the cutting plans. Several studies have also shown that for low birth weight babies (less than 2,000 g) or large babies (more than 4,000 g), the margin of error was greater than 10%, with a tendency to overestimate low birth weight babies and, on the contrary, to underestimate large babies.

    The result is compared to fetal weight estimation curves established by the French College of Fetal Ultrasound. The aim is to detect fetuses that fall outside the norm, located between the 10th and 90th percentile. Fetal weight estimation thus makes it possible to detect these two extremes:

    These two extremes are risky situations for the unborn baby, but also for the mother in the event of macrosomia (increased risk of cesarean section, postpartum hemorrhage in particular).

    Estimating fetal weight is important data for adapting end-of-pregnancy monitoring, the course of delivery and also any neonatal care.

    If during the third ultrasound scan, the estimated fetal weight is lower than the norm, a follow-up ultrasound scan will be performed during the 8th month to monitor the baby's growth. In the event of a threat of premature delivery (TPD), the severity of a possible premature delivery will be estimated based on the term but also on the fetal weight. If the estimated birth weight is very low, the neonatology team will do everything possible to care for the premature baby from birth.

    The diagnosis of macrosomia will also change the management of the end of the pregnancy and that of the delivery. A control ultrasound will be performed during the 8th month of pregnancy in order to make a new estimate of the fetal weight. To reduce the risks of shoulder dystocia (difficulty in delivering the shoulders), brachial plexus injury and neonatal asphyxia, which are greatly increased in the case of macrosomia - by 5% for a baby weighing between 4,000 and 4,500 g and 30% for a baby weighing more than 4,500 g - an induction or a scheduled caesarean section may be proposed. Thus, according to the recommendations of the High Authority of Health:

    If a vaginal delivery is attempted, the obstetric team must be complete (midwife, obstetrician, anesthesiologist and pediatrician) during the delivery considered at risk in the event of macrosomia.

    In the case of breech presentation, the estimated fetal weight is also taken into account when choosing between an attempted vaginal delivery or a scheduled cesarean section. An estimated fetal weight of between 2,500 and 3,800 grams is part of the criteria for acceptability of vaginal delivery. Beyond this, a cesarean section may therefore be recommended.