Leptin and maternal growth during adolescent pregnancy

¹ From the Departments of Obstetrics and Gynecology and of Surgery, University of Medicine and Dentistry of New Jersey—School of Osteopathic Medicine, Stratford, and the Department of Statistics, Temple University, Philadelphia.

² Supported by in part by grant HD18269 from the National Institute of Child Health and Human Development and grant ES07437 from the National Institute of Environmental Health Services.

³ Address reprint requests to TO Scholl, Department of Obstetrics and Gynecology, University of Medicine and Dentistry of New Jersey—School of Osteopathic Medicine, 2 Medical Center Drive, Science Center, Suite 185, Stratford, NJ 08084. E-mail: scholl{at}umdnj.edu.

ABSTRACT  
Background: Maternal growth on the basis of knee height occurs in nearly 50% of pregnant teenagers and is associated with greater gestational weight gain and accrual of subcutaneous fat in the mother but lower fetal growth compared with nongrowing teenagers and mature pregnant women.

Objective: The objective of this study was to determine whether leptin is a biomarker for continued maternal growth.

Design: Leptin concentrations were measured in 162 growing and nongrowing teenage gravidas (aged 18 y) and in mature gravidas (aged 19–29 y) from the Camden Study at 2 time points during pregnancy and 1 time point postpartum.

Results: Growing teenagers had leptin concentrations that increased with gestation and were higher at 28 wk gestation and postpartum than in nongrowing teenagers and mature gravidas. The differences were related to a leptin surge between entry into the study (16.9 wk) and week 28, primarily in still-growing gravidas. Leptin-surge quartiles were associated with higher knee-height velocity and weight gain, increased skinfold thicknesses in late pregnancy (28 wk) and early postpartum (4–6 wk), and changes in postpartum weight and body mass index. For the highest quartile, low birth weight increased >5-fold, fetal growth restriction increased >6-fold, and infant birth weight decreased by 200 g. Gravidas who developed pregnancy-induced hypertension showed a different pattern—higher leptin concentrations at entry and week 28, no difference in the leptin surge, and no postpartum difference in leptin concentration.

Conclusion: A leptin surge by week 28 appears to mark reduced mobilization of maternal fat stores that is associated with maternal growth on the basis of knee height during adolescent pregnancy.

Key Words: Adolescent pregnancy • leptin • growth • birth weight • low birth weight • fetal grow restriction • body composition • skinfold thickness • nutrition • pregnancy-induced hypertension • Camden Study

INTRODUCTION  
Adolescent growth and teenage pregnancy often coincide (1–8). In one study, more than half of teenage females aged 12–18 y had postpartum hand-wrist radiographs that showed an open epiphysis, denoting skeletal immaturity (9). Consequently, many teenage gravidas retain the potential to grow while pregnant. However, when serial measurements of stature are taken, the continued growth of many young gravidas is not clinically apparent (1). Vertebral compression secondary to postural lordosis and gestational weight gain leave the impression that growth has ceased (2).

On the basis of sensitive methods [knee-height measuring device (KHMD)] and a body segment (lower leg) that is less susceptible to "shrinkage" to index growth, 50% of gravidas in the Camden Study grew while pregnant. Maternal growth is associated with larger weight gains, increased fat stores, and greater postpartum weight retention in teenagers compared with nongrowing teenagers and mature pregnant women (3–5). Yet, despite anthropometric changes typically associated with increased fetal size, growing gravidas give birth to infants with lower birth weights (by 150–200 g) than those of infants born to nongrowing gravidas (6). Consistent with this, there are marked reductions in the transfer of micronutrients from the still-growing mother to her fetus (8). There are also changes in uterine artery velocity waveforms on Doppler ultrasound (5). These changes indicate increased placental vascular resistance and are surrogate measures for decreased placental perfusion (10).

Our prior research in Camden, as part of a study in which growth was not measured by KHMD, indicated that a leptin surge during pregnancy (from study entry to week 28) was associated with the constellation of factors also related to maternal growth: higher gestational weight gain and fat accrual but lower infant birth weight (11, 12). Thus, the objective of this study was to determine whether the leptin surge is a biomarker for maternal growth.

SUBJECTS AND METHODS  
Data were derived from the Camden Study, an ongoing research program (1985 to present) on the effects of maternal nutrition and growth during pregnancy in one of the poorest cities in the continental United States (13–15). The institutional review board at the University of Medicine and Dentistry of New Jersey—School of Medicine (UMDNJ-SOM) approved the study. Women with serious nonobstetric problems (eg, lupus, type 1 or 2 diabetes mellitus, seizure disorders, malignancies, drug or alcohol abuse, and psychiatric problems) were excluded.

The 162 subjects included in this analysis were a subsample of pregnant women, studied between 1987 and 1992, whose data were reported previously (4). Subjects were included if they had samples available for leptin assay at 3 time points: entry to care, week 28, and 6 wk postpartum. At each of these visits, plasma was obtained and stored at –70°C in an unopened cryovial until assayed in 1999. Samples were analyzed in duplicate for leptin with use of a kit marketed by Linco Research Inc (St Charles, MO). Intraassay and interassay CVs were <10%.

As described previously (4), maternal growth was measured with the KHMD from entry to care until 6 wk postpartum, an 6-mo interval, and adjusted to a 6-mo velocity. The KHMD measures noninvasively the length of the lower leg, a body segment that is less compressible than is the spine, with a precision of 0.5 mm. A 6-mo knee-height velocity of 1 mm (twice measurement error) excludes women with large soft tissue changes and is used to index growth by KHMD during pregnancy. Growing teenage gravidas aged 18 y were compared with nongrowing teenagers (also aged 18 y) and with mature control subjects aged 19–29 y.

Dietary data (the mean of three 24-h recalls) and socioeconomic, demographic, and behavioral data were obtained as described previously (13–16). Anthropometric measurements were taken on the same schedule. Maternal weight was measured at each visit on a beam balance, height was measured with an anthropometer at entry to prenatal care, and pregravid weight was obtained by recall. It was shown that weight is recalled reliably (r > 0.9) by most people (17, 18). In Camden, the correlation between measured weight (second trimester) and recalled pregravid weight is high (r > 0.9) (19, 20).

Midupper arm circumference and skinfold thicknesses (triceps and subscapular) were measured on the left side at entry, at 28 wk, and postpartum at 4–6 wk (21). Several measurements were derived from the maternal anthropometric measures. Pregravid body mass index (BMI; in kg/m²) was computed; arm muscle area (in cm²) and arm fat areas (in cm²) were computed from triceps skinfold thicknesses and arm circumferences as described previously (21).

Information on duration of gestation, pregnancy complications, and outcome were obtained by abstracting medical records from prenatal care, labor and delivery, and newborn nursery logs as described previously. Pregnancy outcomes included preterm delivery (<37 completed weeks) defined from the mother's last menstrual period, infant low birth weight defined as birth weight <2500 g, and fetal growth restriction defined from birth weight for gestation below the 10th percentile of Brenner's Standard (22), which adjusts for maternal parity, ethnicity, and fetal sex. The major complications of interest included pregnancy-induced hypertension (blood pressure > 140/90 mm Hg).

The leptin surge from entry to week 28 was standardized to a constant time interval and adjusted for the day in gestation, estimated from the last menstrual period, when values were obtained by using least-squares regression. The residuals from regression were added to the overall mean and used in analyses, herein either reported as continuous variables or grouped into quartiles.

The principal statistical methods were analysis of variance, multiple linear regression (for continuous variables), and multiple logistic regression (for dichotomous variables). SAS (version 6.08; SAS Institute Inc, Cary, NC) was used on a Unix operating system. Fixed nested comparisons (least-squares means) were generated to test 3 a priori hypotheses based on our prior studies of maternal growth (1, 3–8, 12) and of leptin during pregnancy (11, 12). The first hypothesis was that the leptin surge was greater for growing gravidas than for the other subjects. Growing gravidas were compared with nongrowing teenagers and with mature control subjects. Bonferroni correction for repeated comparisons was used for these contrasts. The second hypothesis (that the leptin surge, as for maternal growth, was associated with greater gestational weight gain and skinfold thickness) and the third hypothesis (that the leptin surge, as for maternal growth, was associated with a greater risk of infant low birth weight and fetal growth restriction) also were based on our earlier observations. To preserve the experiment-wise error rate, these comparisons were tested sequentially; testing stopped at the first contrast that was not statistically significant. Thus, the highest quartile of the leptin surge (quartile 4) was compared first with the sum of quartiles 1 and 2. If this contrast was significant, quartile 3 was compared with the sum of quartiles 1 and 2. Finally, quartile 3 and quartile 4 were compared with each other.

Separate models were computed for each outcome of interest. Confounding was assessed by comparing adjusted and unadjusted means, logistic regression coefficients, or linear regression coefficients. In logistic models, adjusted odds ratios and their 95% CIs were computed from the logistic regression coefficient and the corresponding covariance matrix; statistical significance for monotonic trend was assessed across the quartiles (23). In certain linear models, repeated-measures analysis of variance (with time as the repeated measure) with least-squares means for the pairwise contrasts, was used to examine leptin by maternal growth during pregnancy and postpartum. In other univariate analyses, Spearman's rank-order correlation coefficients and point-biserial correlation coefficients were computed as measures of correlation.

RESULTS  
Background characteristics of the maternal sample are given in Table 1. When leptin was examined over the course of pregnancy and postpartum, differences between growing teenagers and other gravidas were evident. At entry to care, still-growing teenage gravidas tended to have slightly but not significantly lower leptin concentrations [growers, 1.15 ± 0.10 nmol/L ( ± SEM); nongrowers, 1.27 ± 0.09 nmol/L; and mature control subjects, 1.21 ± 0.15 nmol/L, with adjustment for age, parity, ethnicity, pregravid BMI, number of cigarettes smoked per day, and time point during gestation when the sample was obtained). Between entry and week 28, leptin surged primarily in growers (Table 2). In addition, after control for leptin concentrations at entry and other potential confounding variables, growing gravidas had significantly higher concentrations of leptin at week 28 and postpartum than did nongrowing teenagers and mature pregnant women (Table 2
View this table:
TABLE 1. Maternal background characteristics and Spearman's rank-order correlation coefficients between characteristics and leptin-surge residuals  

View this table:
TABLE 2. Leptin concentrations during pregnancy and postpartum by maternal growth in knee height¹  
On the basis of an analysis of the residual, the adjusted leptin surge had a significant negative rank-order correlation (Table 1) with black ethnicity and entry leptin concentration and there were weaker negative correlations (P < 0.10) with older maternal age (19 y), dietary protein intake (mean of three 24-h recalls), and pregravid BMI. In other words, a larger leptin surge was associated with Hispanic or non-Hispanic white ethnicity and a lower leptin concentration at entry.

Leptin-surge quartiles were associated with a 6-mo knee-height velocity, on average at a level consistent with maternal growth for the highest quartiles (Table 3). As with prior data, leptin-surge quartiles were unrelated to maternal arm fat areas and skinfold thicknesses at entry but were positively and significantly associated with greater arm fat area at 28 wk and 4–6 wk postpartum and with greater increases in fat area and skinfold thicknesses postpartum (Table 3). There was a similar pattern for maternal pregravid weight and BMI. In this instance, gravidas in the highest quartile had significantly higher rates of weight gain over the course of pregnancy, with greater increases in weight and BMI by 6 wk postpartum (Table 3).

View this table:
TABLE 3. Maternal knee-height velocity, weight, and body composition by leptin-surge quartile¹  
As for maternal growth, the leptin-surge quartiles were also associated with increased risks of infant low birth weight (<2500 g) and intrauterine growth restriction. After control for potential confounding variables, there were significant monotonic trends (P < 0.05) for the risk of low birth weight and fetal growth restriction to increase across the quartiles (Table 4). In addition, gravidas in the highest quartile had a >5-fold increase in risk of infant low birth weight and a >6-fold increased risk of intrauterine growth restriction. In each instance, the 95% CIs on the adjusted odds ratios were significant and did not include unity.

View this table:
TABLE 4. Leptin-surge quartiles, low birth weight, and fetal growth restriction¹  
The leptin surge was also associated with lower infant birth weight. After control for duration of gestation and for the potential confounding variables used in the logistic regression models, gravidas in the highest quartile had infants weighing 200 g less (-197.9 ± 99.5 g; P < 0.05) than did gravidas in the 2 lower quartiles.

Finally, using the same methods, we examined leptin concentration and the leptin surge in pregnancy-induced hypertension. Pregnancy-induced hypertension was not associated with maternal growth and, in fact, no growing teenager developed this condition during the current pregnancy (growing teenagers, 0%; nongrowing teenagers, 10.3%; mature control subjects, 7.6%; P = 0.05, chi-square test). Gravidas with pregnancy-induced hypertension had significantly higher concentrations of leptin during pregnancy than did the other subjects, demonstrable at both entry and at week 28. They did not differ significantly in leptin concentration from control subjects postpartum or in the surge by week 28 (Table 5).

View this table:
TABLE 5. Leptin concentrations of women with pregnancy-induced hypertension (PIH) and control group¹  

DISCUSSION  
A leptin surge occurred by week 28, mainly in growing teenage gravidas. On average, leptin showed little change from entry concentrations in mature pregnant women. At 4–6 wk postpartum, leptin concentrations fell to or below entry in mature and nongrowing teenage gravidas, whereas growing teenagers had postpartum leptin concentrations greater than entry concentrations. The surge in leptin associated with maternal growth also paralleled many of the other changes that accompanied maternal growth.

As for maternal growth, the leptin surge was also associated with lower infant birth weight and increased risks of infant low birth weight and intrauterine growth restriction (4–6). These deficits in fetal size occurred despite increases in maternal weight and fatness typically associated with greater fetal growth.

Likewise, the surge in leptin also was associated with positive anthropometric changes that are found in the growing young mother (4, 5). Compared with nongrowing gravidas, weight and skinfold thicknesses typically were lower at entry, were higher during pregnancy, and were retained to a greater extent postpartum in growing gravidas. Similarly, in growing gravidas, leptin concentrations surged during pregnancy and remained above entry concentrations postpartum.

These results replicate those of a smaller independent series of Camden gravidas in whom growth was not measured by KHMD (11, 12). In that series a leptin surge by week 28 was associated with greater maternal weight gain and skinfold thicknesses during pregnancy and lower fetal growth rates (12).

As illustrated by Taggart et al's (24) classic research studies of maternal body composition during pregnancy, subcutaneous fat, as measured by changes in skinfold thickness, accumulates during the first and second trimesters. From late in the second trimester to early in the third trimester, skinfold-thickness measurements begin to decrease at all sites as fat is mobilized to support fetal growth. As for maternal growth, fat accrual later in pregnancy is associated with a paradoxical finding: low infant birth weight despite apparently sufficient maternal energy stores. This observation has been made repeatedly—in Ethiopia (25), possibly in Senegal (26), and in the United States (27). When weight gain was measured it was found to be increased (27). Consistent with studies in human subjects, growing rat dams (28) and young ewes (29) have larger gestational weight gains than do mature pregnant dams as well as lower fetal growth and a higher rate of infant mortality.

Leptin is a polypeptide hormone, a product of the obesity gene and the first hormone found to be released by the adipocyte (30, 31). In pregnant and nonpregnant subjects alike, the leptin concentration usually correlates positively with measures of fatness, including skinfold thickness, weight, and BMI (32, 33). Studies during adolescence associated increases in leptin concentration with the timing of puberty and other hallmarks of adolescence in young women—increases in body fat and the onset of menarche and reproductive function (34–36). Thus, at adolescence, leptin may signal that fat stores are adequate for reproduction. Apart from body fat, the leptin concentration is related to energy intake, but the mechanism is not known (35).

During pregnancy, longitudinal data generally show increases in leptin with gestation and a decline postpartum to concentrations below those measured earlier in pregnancy (37–40). In several of these studies, the peak leptin concentration was obtained at 20–30 weeks' gestation (37, 38, 40), coincident with the peak of maternal fat accumulation (24, 31, 41). In addition, entry leptin is positively related to gestational weight gain and postpartum retained weight (11). Thus, it has been suggested that during pregnancy leptin "senses" the change in maternal fat and that small increases in fat give rise to larger changes in leptin (38).

Factors other than fat regulate the production of leptin. Although leptin increases with gestation and is generated by the placenta (42), the amount produced is not known, and the influence of the hormones of pregnancy is not well understood. In their studies of women undergoing in vitro fertilization, Butzow et al (43) noted a 60% surge in leptin between ovarian suppression and stimulation. This surge was positively correlated with the ovarian response to follicle-stimulating hormone. That is, women with larger leptin surges had fewer ovarian follicles and retrieved oocytes. Likewise, in their study of 29 healthy gravidas, Sivan et al (37) noted a surge in leptin between 12 and 24 wk gestation, which correlated positively with cortisol and negatively with human chorionic gonadotropin (37). The results of in vitro research suggest that estrogen and human chorionic gonadotropin stimulate the release of leptin from cultured adipocytes (37).

Certain complications of pregnancy are associated with higher leptin concentrations during gestation. In pregnancy-induced hypertension, a complication linked to young maternal age, nulliparity, and reduced fetal growth, previous research showed higher leptin concentrations at delivery with no elevation immediately postpartum (44, 45). Increased placental leptin production, in response to placental hypoperfusion, was suspected (44). In our study, gravidas who developed pregnancy-induced hypertension had elevated leptin concentrations throughout pregnancy, at both entry to care and at week 28. Consistent with published studies (44, 45), postpartum leptin concentrations in gravidas with pregnancy-induced hypertension were comparable with those of control subjects. This pattern seemed to differ from the one associated with maternal growth, in which leptin surged during pregnancy and remained elevated postpartum. Significantly higher leptin concentrations during the second trimester of pregnancy were not reported previously for gravidas who ultimately develop pregnancy-induced hypertension, as far as we are aware.

The leptin concentrations reported herein for pregnant women are generally comparable with those of others reported in the literature (33, 38, 40, 45, 46). Although little information is available on the stability of leptin, studies have suggested good short-term (2 mo) stability at 4°C (47) and longer-term (10 y) stability at –70°C (46).

During growth in later adolescence there is continued deposition of fat, which is exacerbated during periods of rapid weight gain (48). During pregnancy, the leptin surge by week 28 appears to mark the increased fat stores that are associated with maternal growth, ie, reserved as part of the young gravidas' own continued development (4). It has been suggested that leptin, in concert with the hormones of pregnancy, plays a role in the partition of metabolic substrate for energy utilization during pregnancy (49). Later pregnancy is characterized by a rise in placental hormones (human placental lactogen, prolactin, progesterone, and estrogen) and insulin resistance. Human placental lactogen, in particular, has an antiinsulin action and is lypolytic, stimulating the release of fatty acids from the mother's adipose tissue (50). During late pregnancy, maternal oxidation of glucose is inhibited and shifted to oxidation of stored fat. In effect, this preserves glucose from the maternal diet for the fetus (51). Reduced lypolysis of maternal fat stores reduces the availability of endogenous fatty acids, thereby increasing glucose utilization by the mother but decreasing availability to the growing fetus (51). The net result is that fetal growth is curtailed while the still-growing young mother is permitted to retain some of the fat accrued as part of her own continued development.

ACKNOWLEDGMENTS  
We thank the staff of the Osborn Family Health Center, Our Lady of Lourdes Hospital, St John the Baptist Prenatal Clinic, and the Women's Care Center, Cooper Hospital, for providing access to patients.

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