Birth weight, prematurity and accuracy of gestational age.
OBJECTIVE
To investigate commonly used birth weight categories in relation to gestational age assignment by menstrual (LMP) or ultrasound dates.


METHODS
A total of 34,249 cases were retrieved from the East Midlands Obstetric Database. Of these, 2281 (6.7%) delivered preterm (< 37 weeks) by ultrasound dating. The percentage of preterm and post-term cases was calculated for birth weight categories from < 1500 g to > 4499 g at 500 g intervals. The incidence of preterm delivery was estimated for birth weights from < 1500 g to 3400 g.


RESULTS
For the 'low birth weight' cut-off of 2500 g, only 64% were actually preterm as assessed by ultrasound, as opposed to 59% as judged by menstrual dates. Nearly 95% of infants weighing over 3000 g are full-term. For birth weights under 2900 g, there is a trend for LMP-derived gestational age to underestimate preterm delivery, by up to 35.5%. By ultrasound dating, 90% of infants under 1800 g are preterm, rising to 98.2% for those under 1500 g.


CONCLUSIONS
As nearly 40% of 'low birth weight' infants are born at term, the old classification of 'low birth weight' should be replaced by gestational age-specific percentile categories. Menstrual dates systematically underestimate the prevalence of preterm delivery. Previous estimates of preterm delivery rates based on LMP data have under-stated their true incidence.
Mongelli
M
M
Department of Obstetrics and Gynaecology, Prince of Wales Hospital, Shatin, Hong Kong.
Gardosi
J
J
D000328:Adult; D001724:Birth Weight; D005260:Female; D005865:Gestational Age; D006801:Humans; D007230:Infant, Low Birth Weight; D007231:Infant, Newborn; D008597:Menstrual Cycle; D007752:Obstetric Labor, Premature / Q000175:diagnosis*; D011247:Pregnancy; D013997:Time Factors; D016216:Ultrasonography, Prenatal