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ORIGINAL RESEARCH |
From the Tropical Metabolism Research Unit, Tropical Medicine Research Institute, The University of the West Indies, Mona, Kingston, Jamaica; and MRC Environmental Epidemiology Unit, Southampton General Hospital, University of Southampton, Highfield, Southhampton, England.
Address reprint requests to: T. E. Forrester, DM, PhD, The University of the West Indies, Tropical Metabolism Research Unit, Tropical Medicine Research Institute, Mona, Kingston 7, Jamaica; E-mail: tesgf{at}infochan.com.
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ABSTRACT |
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METHODS: A total of 712 women were recruited from the antenatal clinic of the University Hospital of the West Indies; 561 fulfilled the study criteria and progressed to delivery. Placental volume and fetal anthropometry (biparietal diameter, head and abdominal circumferences, and femoral length) were measured sonographically at 14, 17, and 20 weeks. The main outcome measures were infant birth and placental weights, length, head, chest, and abdominal circumferences at birth.
RESULTS: Placental volume in the second trimester was positively associated with all birth measurements. Of the fetal measurements at 14 and at 17 weeks, head circumference was the strongest predictor of birth weight (B [slope of the regression line] = .095, P = .014 at 14 weeks; B = .118, P = .012 at 17 weeks), but at 20 weeks, abdominal circumference was the strongest. However, at each age, placental volume was the strongest determinant of birth weight, and improved the prediction based only on fetal measurements. The odds ratio for low birth weight (under 2500 g) increased by 1.68 (95% confidence interval 1.05, 2.69, P = 0.03) for every standard deviation decrease in placental volume at 14 weeks’ gestation.
CONCLUSION: The present study suggests that low birth weight was often preceded by small placental volume in the second trimester. Placental volume may be a more reliable predictor of size at birth than fetal measurements, and may be useful in early identification of the fetus at risk in the perinatal period.
Low birth weight and macrosomia are well known to be associated with disease in the neonatal and subsequent periods of early life. It is less commonly appreciated that in adult life, the risks of hypertension, coronary artery disease, and diabetes mellitus are inversely related to birth weight and newborn anthropometry, and that these relationships operate within the normal range of birth weights.1 Therefore, the size and shape of a neonate might be more relevant to health throughout the life cycle than presently thought.1,2
Assessment of fetal growth has improved with advances in ultrasonography.3,4 Goldenberg et al,5 in a study of serial ultrasound measurements during pregnancy, showed that birth size was only predicted by fetal measurements made in the third trimester. Clapp et al6 reported an association between second-trimester placental volume and birth weight, but did not control for known predictors of birth weight—for example, maternal weight and fetal size. Where sonographic measurements of the placenta have been related to fetal outcome, many studies were not prospective in design. However, Wolf et al,7 in a longitudinal study, described the predictive value of second-trimester placental volume on birth weight, and showed that poor placental growth preceded fetal growth retardation.
If early placental volume improves the ability to predict birth size, then it may enable earlier identification of the fetus at risk, and thus facilitate preparation for management, at least in the neonatal and childhood periods. This longitudinal study was designed to investigate, in women of African origin, the relative ability of placental volume and fetal measurements to predict birth weight. In addition, we sought to identify the earliest point in pregnancy when placental volume proved predictive of birth weight and newborn anthropometry.
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MATERIALS AND METHODS |
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Three individuals, MT, a nurse, and a medical technologist, made all measurements. Two of the three observers made the ultrasound measurements (MT and the technologist). All three were trained to apply the questionnaires and make the measurements with minimum inter- and intra-observer variability. At the start, and at 3-month intervals for the duration of the study, inter-and intra-observer measurement variability was assessed, and training and recertification prescribed for any observer whose scores were not acceptable.2,8
Smoking, alcohol, and drug use were determined from questionnaire responses, and a rating scale based on social amenities and possessions was used to define socioeconomic status.2,8 Maternal weight was measured to the nearest 0.01 kg using a Weylux beam balance (CMS Weighing Equipment, London, UK), and height to the nearest 0.1 cm using a portable stadiometer (CMS Weighing Equipment).
Placental volume and fetal measurements (biparietal diameter, head and abdominal circumference, and femoral length) were measured sonographically (linear probe, ATL Ultramark IV; Advanced Technology Labs, Bothell, WA) at 14, 17, and 20 weeks’ gestation. The method used to measure placental volume required that the entire placenta be seen on the screen. After 20 weeks’ gestation, many placentas are too large for this. Placental volume was measured by identifying and recording on videotape the long axis of the placenta. A continuous recording of the image of the placenta orthogonal to the axis was made by sweeping the probe along the axis at constant velocity. This axis was divided into sections of six equal lengths; the five interior cross-sectional areas were measured and integrated to estimate the placental volume. The average of three measurements was used in analyses. This method was developed and validated by Howe et al.9
Newborn anthropometric measurements were made within 24 hours of delivery. Birth and placental weight were measured with an electronic balance (Soehnle, 800100 Digimail, London, UK); crown heel length was measured with a length board (Holtain, Crymych, UK); head, chest, and abdominal circumferences were measured with a fibreglass measuring tape; gestational age was estimated using the date of the last menstrual period.
Because placental volumes were right skewed, they were transformed to normality by taking square roots, and adjusted for gestational age. The data were analyzed by multiple linear and logistic regression. Newborn measurements were the outcome variables. Maternal weight at booking, weight gain in pregnancy, gender, and gestational age at delivery were controlled for using linear regression techniques because these are known to influence birth weight.10,11 The fetal measurements (biparietal diameter, head and abdominal circumferences, and femoral length) were entered together, with and without placental volume, for each gestational age (14, 17, 20 weeks) in turn. The hypothesis being tested was that placental volume was able to predict birth size at an earlier gestational age and more strongly than fetal measurements.
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RESULTS |
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Less than 1% of the mothers reported the use of alcohol or tobacco, and none admitted to the use of illegal drugs. Mean maternal measurements, placental volume, and birth measurements are given in Table 1
. Mean birth weight, length, and head circumference of the 561 newborns (Table 1) corresponded closely to values obtained from a sample of Jamaican births studied in the 1970s.12
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shows that placental volumes at each of the three gestational ages were positively associated with all birth measurements. The placental volume at 20 weeks’ gestation was most often the strongest predictor.
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shows the ability of the four fetal measurements taken together to predict birth weight. Again, head circumference was the most powerful predictor at 14 and 17 weeks; abdominal circumference, at 20 weeks. Each of the four factors included in the regression as control variables was associated with birth weight. In models with only control variables, the R2 values were 27.6% (14 weeks), 27.0% (17 weeks), and 27.4% (20 weeks).
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). At each of the three gestational ages, placental volume was a strongly significant predictor of birth weight, more so than any of the fetal measurements. Its inclusion improved the fit of the model, which included only fetal measurements (Table 3
versus Table 4, P = .001, P = .001, and P < .001, respectively).
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–4 were found when only the mother’s first antenatal weight was used as a control variable. These analyses were repeated for the other anthropometric measures made at birth (length, head, chest, and abdominal circumferences, and placental weight) with similar results (not shown). None of the anthropometric measures made at birth was associated with socioeconomic status of the mother (not shown).
The earlier analyses were efficient because they used the full information available from the measurement of birth weight, but they did not directly assess determinants of a poor outcome. Therefore, in addition, low birth weight (under 2500 g) and macrosomia (over 4500 g) were used as the dependent variables in multiple logistic regression analyses with the standard set of control variables. The odds ratio (OR) for low birth weight increased by 1.68 (95% confidence interval [CI] 1.05, 2.69, P = .03) for every standard deviation decrease in placental volume at 14 weeks’ gestation. Fourteen of the 27 cases of low birth weight occurred in those whose placental volumes were in the lowest fifth of the distribution at 20 weeks’ gestation (sensitivity 52%, CI 32, 71; specificity 82%, CI 79, 86; OR 3.57). There were only three cases of macrosomia defined as birth weight in excess of 4.5 kg. These occurred in babies whose placental volumes at 20 weeks’ gestation were in the first, second, and fourth fifths of the distribution.
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DISCUSSION |
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There is a growing literature describing a graded increase in the risk of chronic cardiovascular disease and diabetes mellitus in adult life associated with decreasing birth weight and increasing disproportion of head to body size.1 These relationships hold across the normal range of birth weight and proportions. Although newborn anthropometry is not currently included in algorithms to quantify risk of coronary disease, for example, such an application could possibly prove useful in identifying those individuals who should benefit from attaining more stringent goals for coronary disease risk-factor abatement. Ultimately, however, the early diagnosis of fetal growth retardation should be followed by effective interventions to restore normal growth in utero, with the assumption that normal growth patterns confer lower risk of poor health in early life as well as, perhaps, in adulthood.
The placenta is formed at the site of interaction of the decidua basalis and the chorionic villi, and is not seen on ultrasound until the 8th–10th week of gestation when it can usually be identified as an area of increased thickness and echogenicity.17 The chorionic plate of the placenta is formed by the fusion of the amniotic and chorionic mesoderm at approximately 12 weeks’ gestation, and this chorionic plate produces a strong acoustic interface between the amniotic cavity and the fetal surface of the placenta. Before this development, the placenta is not well defined and hence measurement is difficult. The technique used to measure placental volume in this study required that the entire placenta be visualized on the screen.9 Beyond 20 weeks’ gestation, it is difficult to visualize the entire placenta in one field, which makes this technique unsuitable for use in the late second and third trimesters.
This longitudinal study showed clearly that placental volume, early in the second trimester, predicted birth size more accurately than fetal measurements, although fetal abdominal circumference was as strong a predictor at 20 weeks’ gestation. Clapp et al6 and Wolf et al7 also found placental volume to be a stronger predictor of birth weight than fetal measurements made in the second trimester. Goldenberg et al5 reported that evident fetal growth retardation was not detectable from fetal measurements until the third trimester.
Birth weight is influenced by many factors including placental development and growth, and the ability of the placenta to meet the needs of the growing fetus. This longitudinal study suggests that placental volume in the second trimester can predict birth weight and newborn anthropometry and identify the fetus in danger of being low birth weight.
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Footnotes |
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Received November 13, 2000. Received in revised form March 8, 2001. Accepted March 23, 2001.
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REFERENCES |
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2. Thame M, Osmond C, Wilks RJ, Bennett FI, McFarlane-Anderson N, Forrester TE. Blood pressure is related to placental volume and birth weight. Hypertension 2000;35: 662–7.
3. Reece EA, Assimakopoulos E, Zheng X, Hagay Z, Hobbins JC. The safety of obstetric ultrasonography: Concern for the foetus. Obstet Gynecol 1990;76:139–46.
4. Zimmer EZ, Divon MY. Sonographic diagnosis of IUGR-macrosomia. Clin Obstet Gynecol 1992;35:172–84.[Medline]
5. Goldenberg RL, Cliver SP, Neggers Y, Copper RL, DuBard MD, Davis RO, et al. The relationship between maternal characteristics and fetal and neonatal anthropometric measurements in women delivering at term: A summary. Acta Obstet Gynecol Scand 1997;76 Suppl 165: 8–13.
6. Clapp JF, Rizk KH, Appleby-Wineberg SK, Crass JR. Second-trimester placental volumes predicts birth weight at term. J Soc Gynecol Invest 1995;2:19–22.[Medline]
7. Wolf H, Oosting H, Treffers P. Second-trimester placental volume measurement by ultrasound: Prediction of fetal outcome. Am J Obstet Gynecol 1989;160:121–6.[Medline]
8. Forrester TE, Wilks RJ, Bennett FI, Simeon D, Allen M, Osmond C, et al. Fetal growth and cardiovascular risk factors in Jamaican schoolchildren. Br Med J 1996;312: 156–60.
9. Howe D, Wheeler T, Perring S. Measurement of placental volume with real time ultrasound in mid-pregnancy. J Clin Ultrasound 1994;22:77–83.[Medline]
10. Kramer MS. Determinants of low birth weight: Methodological assessment and meta-analysis. Bulletin of the World Health Organization 1987;65:663–737.[Medline]
11. Abrams BF, Laros RK. Prepregnancy weight, weight gain, and birth weight. Am J Obstet Gynecol 1986;154:503–9.[Medline]
12. Lowry MF, Bailey R. The size of the Jamaican newborns from 27–42 weeks gestation. W I Med J 1978;27:137–46.
13. Hoogland HJ, de Haan J, Martin CB. Placental size during early pregnancy and fetal outcome: A preliminary report of a sequential ultrasonographic study. Am J Obstet Gynecol 1980;138:441–3.[Medline]
14. McCormick MC. The contribution of low birth weight to infant mortality and childhood morbidity. New Engl J Med 1985;312:82–90.[Abstract]
15. Vilar J, de Onis M, Kestler E, Bolanso F, Cerezo R, Bernedes. The differential neonatal mortality of the intrauterine growth retardation syndrome. Am J Obstet Gynecol 1990;163:151–7.[Medline]
16. Walther FJ, Ramaekers LH. Neonatal morbidity of SGA infants in relation to their nutritional status at birth. Acta Paediatr Scand 1982;71:437–40.[Medline]
17. Hadlock FP, Athey PA. Sonography of the placenta. In: Larvey JP, ed. The human placenta: Clinical prospectives.Rockville, MD: Aspen Publishers, 1987:101–6.
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