Postterm infants: Too big or too small? Frances H. McLean, BScN, Mark E. Boyd, MD, Robert H. Usher, MD, and Michael S. Kramer, MD* Montreal, Quebec, Canada Concern over the postterm pregnancy has shifted from that of the difficult delivery of an excessively large fetus to the current concern with death in utero of an undernourished, small-for-date fetus. Studies of postterm pregnancy before the availability of ultrasonography may have included a large proportion of erroneous menstrual dates. The present study of 7000 infants was undertaken to reassess fetal growth in postterm pregnancies in which the expected date of confinement from last normal menstrual period dating was confirmed (± 7 days) by early ultrasonography. Results show a gradual shift toward higher birth weight and greater crown-heel length and head circumference between 273 and 300 days of gestational age. No evidence of postterm weight loss or lower weight for length could be demonstrated. Concern in postterm pregnancy should be for fetal macrosomia, not for intrauterine growth retardation. (AM J OBSTET GVNECOL 1991 ;164:619-24.)

Key words: Fetal growth, postterm The controversy as to whether postterm infants tend to be "too big" or "too small" remains unresolved. In 1902, Ballantyne described the postterm infant as excessive in size and the essential problem with delivery as cephalopelvic disproportion. I There have since been several studies that reported increased rates of macrosomic infants in pregnancies that progressed past term. Z.6 In contrast, Clifford 7 described the long, thin postterm infant with "the picture of recent weight loss" that he attributed to placental dysfunction, which led to fetal wasting, hypoxia, meconium release, and death. Since this description, there have been additional reports of this "placental dysfunction syndrome." The current concern of the postterm pregnancy usually relates more to fetal jeopardy from deprivation of oxygen and nutrition than to difficult delivery. The conflicting opinions about the size of the postterm infant, the need for increased surveillance before labor, and the induction of labor have been discussed in the literature but with differing conclusions. B• g There appears to be no doubt that the "placental dysfunction syndrome" can occur, but there is no evidence that it occurs only or even more frequently in postterm pregnancies than it does at or before term. One problem involves determining which infants are From the Departments of Obstetrics and Gynecology, Pediatrics, and Epidemiology and Biostatistics, McGill University Faculty of Medicine, and the Royal Victoria Hospital. Received for publication February 6, 1990, revised August 17, 1990, accepted August 29, 1990. Reprint requests: Frances H. McLean, Neonatal Unit, Women's Pavilion, Royal Victoria Hospital, 687 Pine Ave. W., Montreal, Quebec, Canada H3A lAl. *Dr. Kramer is a senior investigator of the Fonds de la Recherche en Sante du Quebec. 6/1/25061

actually postterm. Previous studies considered infants to be postterm on the basis of the expected date of confinement calculated from the mother's last normal menstrual period. It has recently been shown that in only a small minority of infants considered postterm by last normal menstrual period could the menstrual age be confirmed by early ultrasonography.lO· II Another possibility is that the postterm population may include both types of infants (some too big and others too small or too scrawny); if so, population distribution by birth weight must be analyzed and not determined from mean values. The present analysis of fetal growth characteristics and body proportions was therefore undertaken on a population in whom gestational age from last normal menstrual period was confirmed to within 7 days by early ultrasonography. Confirmed "postterm" infants (~42 completed weeks) are compared with those born at 39,40, and 41 weeks from the onset of the last normal menstrual period.

Population and methods The infants studied were delivered between Jan. 1, 1978, and March 31, 1986, at the Royal Victoria Hospital, Montreal, and were previously reported with respect to obstetric management and fetal outcome. 10. 12 Criteria for inclusion in the study were known date of onset of the last normal menstrual period, early (usually 16 to 18 weeks) ultrasonography measurement from which an estimated date of confinement was entered into the McGill Obstetric and Neonatal Database, 13 concordance (± 7 days) of the estimated date of confinement on the basis of menstrual history and early ultrasonography, and delivery at or after 39 weeks (273 days) from onset of the last normal menstrual period.

619

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February 1991 Am J Obstet Gyneco1

Table I. Birth weight by gestational age Gestational age Completed weeks

39 40 41 42

FL P Value

I

Primiparous women Days

No. of infants

273-279 280-286 287-293 294-300

1279 1208 781 198

I

Multiparous women

Birth weight (gm, mean ± SD)

3354 3498 3604 3711

±

405

±

414

± 409 ± 435

150.60

Postterm infants: too big or too small?

Concern over the postterm pregnancy has shifted from that of the difficult delivery of an excessively large fetus to the current concern with death in...
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