Maternal weight and body composition in the first trimester of pregnancy
Abstract
Objective. Previous studies on weight gain in pregnancy suggested that maternal weight on average increased by 0.5–2.0 kg in the first trimester of pregnancy. This study examined whether mean maternal weight or body composition changes in the first trimester of pregnancy. Design. Prospective observational study. Population. We studied 1,000 Caucasian women booking for antenatal care in the first trimester of pregnancy. Setting. Large university teaching hospital. Methods. Maternal height and weight were measured digitally in a standardized way and Body Mass Index (BMI) was calculated. Maternal body composition was measured using segmental multifrequency Bioelectrical Impedance Analysis (BIA). Sonographic examination confirmed the gestational age and a normal ongoing singleton pregnancy in all subjects. Main outcome measures. Maternal weight, maternal body composition. Results. The mean BMI was 25.7 kg/m2 and 19.0% of the women were in the obese category (≥30.0 kg/m2). Cross-sectional analysis by gestational age showed that there was no change in mean maternal weight, BMI, total body water, fat mass, fat-free mass or bone mass before 14 weeks gestation. Conclusions. Contrary to previous reports, mean maternal weight and mean body composition values remain unchanged in the first trimester of pregnancy. This has implications for guidelines on maternal weight gain during pregnancy. We also recommend that calculation of BMI in pregnancy and gestational weight gain should be based on accurate early pregnancy measurements, and not on self-reported or prepregnancy measurements.
Introduction
Obesity in pregnancy has emerged as a major concern in modern obstetrics for two reasons. Firstly, there is strong epidemiological evidence that obesity is associated with increased pregnancy complications (1–6). There is, for example, an increase in maternal complications such as diabetes mellitus, pre-eclampsia/hypertension, cesarean delivery and surgical complications. There is also an increase in fetal complications such as preterm birth, congenital anomalies, intrauterine growth abnormalities and in the longer term, childhood obesity.
Secondly, based on a Body Mass Index (BMI) World Health Organization (WHO) categorization of ≥30.0 kg/m2, the prevalence of maternal obesity is high and rising (5,6). One in five women booking for antenatal care in our hospital in 2008 was obese (7). The Centre for Maternal and Child Enquires (CMACE) in Britain has selected obesity in pregnancy as its principal project with a maternal health focus for 2008–2011 because obesity is associated with increased maternal and perinatal mortality.
In the United States of America concerns are so great that the Institute of Medicine (IOM) has recently published new guidelines for weight gain during pregnancy (8). Upward trends in both actual and recommended gestational weight gain had been reported which has lifelong implications for the woman as well as for the pregnancy itself (9). The new 2009 guidelines are based on the WHO BMI categories with a specific, relatively narrow range of recommended gain for obese mothers of 5.0–9.0 kg (8).
It has been reported that body fat increases begin early in pregnancy and the new IOM guidelines assume that weight increases in the first trimester (8, 9). Previous studies on maternal obesity, however, were usually based on self-reporting of BMI, which is unreliable (7). Furthermore, BMI is only a surrogate marker for adiposity and has many limitations (10).
Recent advances in Bioelectrical Impedance Analysis (BIA) allow direct measurements of fat and fat distribution to be made during pregnancy (11). Outside of pregnancy, BIA has been shown to be non-invasive, relatively inexpensive, does not expose to ionizing radiation, has very limited between observer variations and can be easily performed in healthy subjects (12).
The objective of this cross-sectional study was to determine if mean maternal weight or body composition measurements change in the first trimester of pregnancy.
Materials and methods
Between July 2008 and June 2009 we enrolled 1,000 women with a singleton pregnancy during the first trimester of pregnancy. An ultrasound examination confirmed gestational age and a normal ongoing pregnancy. To eliminate race as a variable, we confined the study to Caucasian women. We excluded women with diabetes mellitus, cardiovascular disease and hyperemesis requiring intravenous hydration. Each woman was given an information leaflet and written consent was obtained. No woman refused to participate. The study was approved by the Hospital Research Ethics Committee.
Height was measured using a Seca wall-mounted digital meterstick with the woman standing erect in her bare feet. Weight and body mass composition were measured using segmental multifrequency Bioelectrical Impedance Analysis (Tanita MC 180MA, Tokyo, Japan). The woman was asked to stand on the floor plate. Thirty seconds was allowed for the system to stabilise. She then held the hand paddles by her side for 20 seconds while the system calculated maternal body composition. The BMI was calculated based on measurement of height and weight by a single observer (NO'C). The ultrasound examination and maternal measurements were performed on the same day. The measurements taken on the first antenatal visit only were included.
Data was expressed as mean ± standard deviation. We used Levene's statistic to test for homogeneity of variance. A normal probability plot was used to check for distribution of the data. Analysis of variance (ANOVA) was used to ascertain differences between the gestational groups. A p-value < 0.05 was considered significant. The statistical software package SPSS 15.0 (SPSS Inc., Chicago, IL, USA) was used for data analysis.
Results
Of the 1,000 women, 49.2% were primigravidas and 50.8% were multigravidas. Of the multigravidas, 27.5% had one child, 15.5% had two children and 7.8% more than two. The mean age was 28.2 years (range 16–44). The mean BMI was 25.7 kg/m2 (range 16.4–47.1), 3.1% were underweight (<18.5 kg/m2), 50.2% were in the normal weight category (18.5–24.9 kg/m2), 27.7% in the overweight category (25.0–29.9 kg/m2) and 19.0% of the women were in the obese category (≥30.0 kg/m2). The mean weight was 69.9 kg (range 40.8–146.0). Table 1 shows the maternal weight, BMI and body composition values. Table 2 shows the mean maternal parameters analyzed by gestational age. Statistical analysis showed no differences in mean maternal weight or body composition values in the first trimester of pregnancy as gestational age advances. Statistical analysis also showed no differences in the mean age, parity and educational status of the women at each gestational age (Table 3).
| Mean | SD | |
|---|---|---|
| Weight (kg) | 69.1 | 14.7 |
| Body mass index (kg/m2) | 25.7 | 5.3 |
| Total body water (%) | 34.0 | 4.3 |
| Bone mass (kg) | 2.4 | 0.3 |
| Fat free mass (kg) | 46.4 | 6.0 |
| Fat mass (kg) | 22.7 | 9.6 |
| Gestation (weeks) | 5–7 (n = 56) | 8 (n = 52) | 9 (n = 69) | 10 (n = 135) | 11 (n = 214) | 12 (n = 300) | 13 (n = 174) |
|---|---|---|---|---|---|---|---|
| Weight (kg) (SD) | 71.4 (17.5) | 70.8 (14.4) | 67.1 (13.0) | 70.1 (15.1) | 69.5 (16.0) | 68.3 (13.9) | 68.6 (14.1) |
| Body mass index (kg/m2) (SD) | 26.8 (6.3) | 26.0 (5.4) | 25.2 (4.7) | 25.8 (5.5) | 26.0 (5.9) | 25.5 (5.0) | 25.6 (5.0) |
| Total body water (%) (SD) | 34.2 (4.7) | 34.2 (3.7) | 32.6 (4.4) | 33.6 (4.3) | 33.5 (4.6) | 33.1 (4.0) | 33.3 (4.2) |
| Bone mass (kg) (SD) | 2.4 (0.3) | 2.4 (0.3) | 2.3 (0.3) | 2.4 (0.3) | 2.4 (0.3) | 2.3 (0.3) | 2.4 (0.3) |
| Fat free mass (kg) (SD) | 47.6 (6.6) | 47.5 (5.3) | 45.2 (5.9) | 46.8 (6.0) | 46.5 (6.4) | 45.9 (5.7) | 46.3 (5.9) |
| Fat mass (kg) (SD) | 24.3 (12.0) | 23.3 (9.7) | 21.8 (8.2) | 23.3 (9.8) | 23.0 (10.4) | 22.3 (9.1) | 22.4 (9.1) |
| Gestation (weeks) | 5–7 (n = 56) | 8 (n = 52) | 9 (n = 69) | 10 (n = 135) | 11 (n = 214) | 12 (n = 300) | 13 (n = 174) |
|---|---|---|---|---|---|---|---|
| Age (years) | 29.6 | 27.7 | 26.9 | 27.9 | 28.2 | 28.3 | 28.7 |
| Parity | 1.1 | 0.9 | 0.8 | 1.0 | 0.9 | 0.8 | 0.8 |
| Completed education (years) | 18.5 | 18.5 | 18.4 | 18.2 | 19.0 | 18.9 | 18.9 |
Discussion
We found, to our surprise, that mean maternal weight and thus, mean BMI did not change in the first trimester. Bioelectrical Impedance Analysis also showed no change in mean maternal body composition. In particular, mean body fat measurements remained unchanged. These findings indicate that changes in maternal weight or body composition in pregnancy usually occur after the first trimester. Therefore, accurate measurement of weight or body composition at any time in the first trimester may be used as a baseline for subsequent comparison.
In 1990, the Institute of Medicine (IOM) in the United States of America published new recommendations for total weight gain in pregnancy which exceeded previously recommended weight gains (13, 14). This was in response to concerns about adverse clinical outcomes associated with poor maternal weight gain. Subsequently, the National Academy of Science issued a Workshop Report in 2007 on the influence of pregnancy weight on maternal and child health (15). The main concern was now excessive maternal weight gain with a 33% prevalence rate of obesity in adult American women. The Report also recognized that gaps in knowledge have emerged about maternal weight in pregnancy. There is, for example, a paucity of information about maternal weight and body composition in the first trimester of pregnancy (15).
In a study of 2,994 nonobese white women delivered between 1980 and 1990, a first trimester weight gain of 2.1 kg (SD 3.3) was reported (16). The weight and height were, however, self-reported and were not measured at the first antenatal visit. Sonographic first trimester dating scans were not standard. First trimester weight gain was estimated as total gain minus second and third trimester gains.
The same group also reported on factors associated with the pattern of maternal weight gain during pregnancy in 7,587 multiracial women (17). Again, BMI calculation was based on self-reporting and weight gain was estimated. They concluded that gain was slowest in the first trimester at 0.169 kg/week on average.
They subsequently reported on a multiracial group confined to 4,218 women with a good pregnancy outcome (18). Again, BMI was self-reported and gestational age was based primarily on the reported date of the last menstrual period. Only 4% of the group studied were obese. During the study period, weight gain was encouraged. The mean weight gain attributed to the first trimester was 2.19 kg (SD 3.47) in normal weight women but, the authors acknowledged that these findings could be due to errors in recall of prepregnancy weight.
In a study of 5,766 selected women, predominantly of Hispanic origin (80%) from a population at high risk of preterm delivery, weight gain in the first trimester was estimated at 0.89 kg (SD 2.6) by 12 weeks gestation (19). The results were based on self-reporting of BMI. Gestation was determined from the onset of the last menstrual period. The calculation of weight gain was estimated and based on a number of mathematical assumptions.
In a recent meta-analysis of the impact of maternal BMI status on pregnancy outcomes, 919 cohort studies were identified of pregnant women with anthropometric measurements recorded before 16 weeks gestation (3). Only 49 were deemed to be of high enough quality for inclusion in the meta-analysis. Only four of the 49 measured BMI, and none of the four measured BMI during the first trimester. Sonographic confirmation of dates was not standard practice. Another systematic review on the natural history of postpartum weight retention stated that there is already significant maternal weight gain in the first trimester but, acknowledged that the studies supporting this statement were not based on first trimester measurements (20).
More recently, the Institute of Medicine has further revised the guidelines on weight gain during pregnancy (8). The new recommendations, based on WHO BMI categories, assume a 0.5–2.0 kg (1.1–4.4 lbs) weight gain in the first trimester (8). This assumption is based on three publications that we have cited of Californian women delivered in the 1980s (17–19). None of these studies measured maternal weight in the first trimester.
Maternal weight gain attributed to the first trimester may be explained by the inaccuracies of self-reporting. We have recently shown that self-reporting in the first trimester underestimates maternal weight by 1.7 kg (SD 2.4) (7), which is similar in magnitude to the weight gain attributed in the past to the first trimester. Also, previous studies on gestational weight gain often used prepregnancy weight measurements (3), which may bias findings because reliable data is not available in about half of the women whose pregnancy is unplanned (21, 22).
Our findings challenge previous reports on weight gain in the first trimester of pregnancy. The strengths of our study are that the maternal values were measured and not self-reported, and that the gestational age was confirmed by ultrasound. The findings on weight are also underpinned by the absence of changes in maternal body composition. The results have implications for recommendations to women on gestational weight gain and for research on maternal obesity.
Observational data has shown that women who gain within the IOM (1990) guidelines experienced better outcomes of pregnancy that those who do not (8). Accordingly, it has been recommended that women's weight should be charted during pregnancy and that individual advice should be given about both diet and physical activity. Our study shows that accurate measurement of maternal weight and BMI in the first trimester may be used as a baseline for assessing both gestational weight gain and BMI.
Acknowledgments
Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.
