For future parents, estimating fetal weight on ultrasound allows you to imagine this long-awaited baby a little better. For the medical team, this data is essential for adapting the pregnancy follow-up, the delivery method and the care of the baby at birth.

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

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

• the cephalic perimeter (PC or HC in English);
• the bi-parietal diameter (BIP);
• the abdominal perimeter (PA or AC in English);
• the length of the femur (LF or FL in English).

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.

There are about twenty calculation formulas but in France, those of Hadlock are the most used. There are several variants, with 3 or 4 biometric parameters:

• Log10 EPF = 1.326 – 0.00326 (AC) (FL) + 0.0107 (HC) + 0.0438 (AC) + 0.158 (FL)
• Log10 EPF = 1.3596 + 0.0064 PC + 0.0424 PA + 0.174 LF + 0.00061 BIP PA – 0.00386 PA LF

The result is indicated on the ultrasound report with the mention “EPF”, for “Estimation of fetal weight”.

However, the result obtained remains an estimate. Most of the formulas have been validated for birth weights of 2 to 500 g, with a margin of error compared to the actual birth weight ranging from 4 to 000% (6,4), due to part to the quality and precision of the cutting plans. Several studies have also shown that for low weight babies (less than 10,7 g) or large babies (over 1 g), the margin of error was greater than 2%, with a tendency to overestimate babies. of small weight 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 (3). The goal is to screen the fetuses out of the norm, located between the 10 ° and the 90 ° percentile. The estimation of the fetal weight thus makes it possible to detect these two extremes:

• hypotrophy, or low weight for gestational age (PAG), that is to say a fetal weight below the 10th percentile according to the gestational age given or a weight below 2 g at term. This PAT can be the consequence of a maternal or fetal pathology or of a uteroplacental anomaly;
• a macrosomia, or “big baby”, that is to say a baby of fetal weight greater than the 90th percentile for the given gestational age or even with a birth weight greater than 4 g. This monitoring is important in the case of gestational diabetes or pre-existing diabetes.

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, bleeding during delivery in particular).

The estimation of the fetal weight is an important data to adapt the follow-up of the end of pregnancy, the progress of the childbirth but also the possible neonatal care.

If on the third ultrasound the estimate of the fetal weight is lower than the norm, a follow-up ultrasound will be performed during the 8th month to monitor the growth of the baby. In the event of a threatened premature birth (PAD), the severity of a possible premature birth will be estimated according to the term but also to the fetal weight. If the estimated birth weight is very low, the neonatal team will put everything in place to take care of the premature baby from birth.

The diagnosis of a macrosomia will also change the management of late pregnancy and childbirth. A follow-up ultrasound will be performed during the 8th month of pregnancy in order to make a new estimate of the fetal weight. To reduce the risk of shoulder dystocia, brachial plexus injury and neonatal asphyxia, greatly increased in macrosomia – by 5% for a baby weighing between 4 and 000 g and 4% for a baby over 500 g (30) – induction or scheduled cesarean section may be offered. Thus, according to the recommendations of the Haute Autorité de Santé (4):

• in the absence of diabetes, macrosomia in itself is not a systematic indication for scheduled cesarean section;
• the scheduled cesarean section is recommended in the event of an estimated fetal weight greater than or equal to 5 g;
• due to the uncertainty of the estimate of the fetal weight, for a suspicion of macrosomia between 4 g and 500 g, the scheduled cesarean section must be discussed on a case-by-case basis;
• in the presence of diabetes, scheduled cesarean section is recommended if the fetal weight is estimated to be greater than or equal to 4 g;
• due to the uncertainty of the estimate of the fetal weight, for a suspicion of macrosomia between 4 g to 250 g, the scheduled cesarean section must be discussed on a case-by-case basis, taking into account other criteria related to the pathology and obstetrical context;
• the suspicion of macrosomia is not in itself a systematic indication for a planned cesarean section in the event of a scarred uterus;
• If macrosomia is suspected and a history of shoulder dystocia complicated by elongation of the brachial plexus, a scheduled cesarean section is recommended.

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

In case of breech presentation, the estimate of the fetal weight is also taken into account when choosing between an attempt at vaginal route or a scheduled cesarean section. A fetal weight estimated between 2 and 500 grams is part of the acceptability criteria for the vaginal route established by the CNGOF (3). Beyond that, a cesarean may therefore be recommended.