Fetal Growth

Definition

Currently, there is no consensus on the definition of macrosomia or large for gestational age (LGA) fetuses. The term LGA is often used to describe fetuses with an estimated fetal weight (EFW) greater than the 90th centile for gestation. Macrosomia is used to describe a newborn with birthweight above a certain limit – usually 4 or 4.5 kg. The American College of Obstetricians and Gynaecologists uses a birthweight of >4.5 kg to define macrosomia, as both maternal and perinatal complications begin to increase above that fetal weight.

Causes

Many of the risk factors for macrosomia are fixed. These include male sex, multiparity, increased maternal age at first pregnancy, increased maternal height, white race and gestational age greater than 41 weeks. A previous macrosomic baby is a strong risk factor for high birthweight.

Other risk factors that can be controlled or modified include pre-pregnancy Body Mass Index (BMI), which is independent of diabetes/impaired glucose tolerance and excess weight gain in pregnancy (increase in BMI of >25%).

Diagnosis

Clinical and ultrasound methods are inaccurate in the prediction of macrosomia because the measurement of the height of the uterus is subject to significant variation. Ultrasound methods use either a single measurement, such as abdominal circumference, or calculate EFW from a combination of measurements. There is no difference in any of these ultrasound methods in the prediction of a macrosomic baby at birth.

A positive test result is a more accurate way of ruling in macrosomia than a negative test is at excluding it. The accuracy of predicting macrosomia at birth can be improved by combining amniotic fluid index ( > 60th centile) with EFW ( > 71st centile) in the third trimester (positive predictive value 85%).

When compared, clinical and ultrasound methods have similar and limited power to predict fetal weight >4 kg. In both methods, the areas under the receiving operator curve are between 0.81 and 0.95, which is defined as useful in statistical terms. However, the mean absolute error in estimating weight of macrosomic babies is 250 g to 500 g in most studies using both clinical and sonographic methods.

Serial sonographic measurements may improve the positive predictive value, but can be impractical in terms of time and cost. Whichever ultrasound formula is used to calculate EFW, the accuracy decreases as the birthweight increases. There is a clear need for improved methods to estimate fetal weight and body proportions.

Clinical Importance – Perinatal Consequences

Infants with a birthweight between 4000 g and 4499 g are not at increased risk of adverse outcome compared with birthweights of 3500 g to 3999 g. However, a birthweight of 4500 g to 4999 g has significantly increased risks of perinatal morbidity and mortality, birth injury (shoulder dystocia), meconium aspiration and Caesarean delivery. Babies with a birthweight of >5000 g are at even higher risk as they are twice as likely to die from sudden infant death syndrome as babies of normal birthweight.

Long-Term Consequences of Macrosomia

Fetal macrosomia is associated with long-term health problems including adult obesity, hypertension and impaired glucose tolerance.

Management

Ultrasound plays a key role in the diagnosis and management of fetal macrosomia, but it is important to use it appropriately with a thorough clinical assessment including maternal BMI, weight gain and past obstetric history.

There is little evidence to support the routine use of either induction of labour or elective Caesarean section to prevent adverse outcome in non-diabetic women with an ultrasonically diagnosed LGA baby of less than 5000 g. However if the EFW is calculated to be >5000 g there is evidence to support elective Caesarean section.