The objective of the study was to assess the factors that affect fetal growth and birthweight, and to derive coefficients for a customized growth chart applicable in an American population.

This was a prospective cohort study of 35,235 pregnancies. Coefficients for physiological and pathological variables were derived by backward multiple regression.

The expected birthweight at 40.0 weeks for a standard-size primiparous mother of European origin in an uncomplicated pregnancy was 3453.4 g, very similar to the standardized birthweight observed in other populations.

• Physiological coefficients were derived for
• maternal height,
• weight,
• parity,
• ethnic origin,
• and sex of the baby.
• Smoking,
• history of preterm delivery,
• and hypertensive diseases in the current pregnancy all had negative effects on birthweight,
• whereas babies of diabetic mothers weighed more.
• Low as well as high body mass index was associated with birthweight deficit at term.

Coefficients that allow determination of the customized growth potential, individually adjusted and excluding known pathological factors, have been derived. Babies of obese mothers have an increased risk of not reaching their fetal growth potential.

Key words: birthweight, customized growth charts, fetal growth, growth potential

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• ^{• Abstract}
• ^{• Materials and Methods}
• ^{• Study population}
• ^{• Statistical analysis}
• ^{• Results}
• ^{• Comment}
• ^{• References}
• ^{• Copyright}

Accurate assessment of intrauterine growth is an essential part of antenatal care and perinatal research. It requires a standard that can be individually adjusted or customized to reflect the growth potential of the fetus in each pregnancy.1

To determine the customized growth potential, a predicted weight at term for a pregnancy in optimal conditions is firstly calculated, using adjustment coefficients derived from the local population.2

Physiological or constitutional variables such as maternal size, parity and ethnic origin are adjusted for, whereas pathological factors such as smoking, hypertensive diseases and diabetes are excluded, even if they are known to be present, to set the expected standard so as to better recognize if fetal growth has been affected.

The predicted “term optimal weight” is combined with a proportionality function derived from an ultrasound-based fetal weight curve to determine the optimal and normal range of fetal weight for each point in gestation.2

Such a customized standard has been found to improve the distinction between normal and abnormal growth and to enhance our understanding of the factors associated with fetal growth restriction.3, 4, 5, 6, 7

It is recommended by Royal College of Obstetricians and Gynaecologists Guidelines8 and is already in widespread use clinically and in ongoing research.

There has recently been a call for customized growth charts to be adopted by obstetricians in the United States.9

Locally derived standards with appropriate coefficients for adjusting the expected term weight according to physiological variables have been published for maternity populations in the United Kingdom,2 New Zealand,10 France,11 Spain,12 and Australia.13

The main purpose of this study was to derive coefficients that can be used to determine a customized fetal growth potential in an American population

Jason Gardosi, MD, FRCOG, Andre Francis, MSc

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http://www.ajog.org/article/S0002-9378(09)00429-3/fulltext

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