Fetal macrosomy refers to the condition of a baby who weighs more than 4 kg during childbirth

Macrosomy Fetale

Fetal macrosomy refers to the condition of a baby who weighs more than 4 kg during childbirth. To particularly important weight, body dimensions should be added in the 90th percentile of maximum normal values defined by pediatricians, and visible by ultrasound. This medical condition can lead to complications for both mother and child. Macrosomy is between 5 % and 10 % of pregnancy.


Doctors usually estimate the weight of the fetus in the third ultrasound (circa 32 SA). When diagnosed above the curves, an extra ultrasound is carried out around 36/37 SA to monitor the evolution. But, ′′ The ultrasound is far from giving accurate measure and the errors are in the order of 10 %. Another exam provides an indication of the weight of the fetus, it is the measure of the uterine height (the distance from pubic to womb, so belly size). Again, this exam can only give an estimate.


Maternal diabetes, type 1 or 2 pre-existing to pregnancy or gestational (appeared during pregnancy), is the leading cause of fetal macrosomy. Other risk factors are also identified: obesity of the future mother, a significant weight gain during pregnancy, being over 40 years old, giving birth more than 4 times. Future moms with gestational or pre-existing diabetes receive special support from a dietician / nutritionist or diabetologist / endocrinologist (followed by diet, recommendation for physical activity…). They are more monitored by their doctor ( more ultrasound etc) to limit fetal weight gain.

What are the risks of fetal macrosomy?

Fetal macrosomy can cause complications at childbirth. On the mom side, caesarean section and forceps use are more frequent as well as perineal tearing. Baby side, the worst complication is shoulder dystocia. In this case, during the expulsion, the shoulders of the fetus have difficulty getting through the maternal pool or even remain blocked. The ′′ fake ′′ shoulder dystocia is characterized by the poor positioning of one of the shoulders. The obstetrician manages to release it through external maneuvers, including the toppling of the maternal basin. The ′′ real ′′ dystocia, it is rare, cannot anticipate itself and requires the obstetrician to do an endo-uterine manoeuvre (inside the womb) to unlock the baby’s shoulders very quickly. This is an emergency.

When the foetus weighs more than 4,5 kilos, morbidity (number of sequelae or complications) and death (number of deaths) are also higher.

Special delivery mode in case of fetal macrosomy

Caesarean section: When the baby’s estimated weight exceeds 5 kilos, a c-section is recommended. Same thing when the future mother suffers from diabetes and the foetus exceeds 4,5 kilos.

The trigger: This method of childbirth is often envisaged around 38/39 SA when the estimated foetus weight exceeds 95th percentile. Having the mother to be given birth two to three weeks earlier than the term announced, the foetus is avoided growing more and the risk of shoulder dystocia.

It has not been shown that the outbreak of work for presumptive fetal macrosomy in non-diabetic women alter the risk of maternal or neonatal morbidity, but the ability of studies included to show a difference in rare events is limited. Greater tryouts will be needed to answer this question.