Misperceived pre-pregnancy body weight status predicts excessive gestational weight gain: findings from a US cohort study

Study subjects were recruited into Project Viva at their first prenatal visit from one of eight urban and suburban obstetric offices associated with a multispecialty group practice in eastern Massachusetts from1999 to 2002 [31]. Eligibility criteria included fluency in English, gestational age <22 weeks, and a singleton pregnancy. All mothers provided written informed consent, and all procedures were in accordance with ethical standards for human experimentation. Institutional review boards of participating sites approved the study.

Of the 2128 women who delivered a live singleton infant, we restricted our analysis to include the 1835 women who were normal weight, overweight, or obese body mass index (BMI). We did not include underweight women as they were few in number and are at low risk of excessive weight gain during pregnancy [21]. We then excluded women with missing information on perceived body weight (n = 284) or who had no measurement of gestational weight gain recorded within the 4 weeks preceding delivery (n = 14), leaving 1537 women for this analysis. Compared with the 298 women who were not included, the 1537 women in this analysis were somewhat older (mean of 32.3 versus 30.0 years), more likely to be white (72% versus 36%), college educated (68% versus 41%), and normal weight (67% versus 52%), but had only slight differences in rates of average (62% versus 64%) and overweight (37% versus 36%) pre-pregnancy weight status perception.

Mothers reported pre-pregnancy weight and height by questionnaire at study enrollment. We calculated BMI (kg/m²) from these self-reported measures and classified women as normal weight (BMI 19.8–26.0 kg/m²) or overweight/obese (BMI > 26.0 kg/m²) according to Institute of Medicine criteria [11]. Validation of self-reported weights with clinically measured weights among a sample of 170 women in this cohort revealed a tight correlation (r = 0.99), with a mean systematic underreport of 1 kg that did not vary by maternal race/ethnicity, gestational age at enrollment into the study, or weight itself [9]. Moreover, the very high correlation indicates that ranking of individuals is well preserved.

We assessed perceived body weight status via early pregnancy questionnaire (approximately 10 weeks gestation) by asking participants, "How would you classify your weight just prior to this pregnancy?" Responses ranged from (1) markedly overweight to (5) markedly underweight. A similarly structured perceived weight status question is asked of non-pregnant women and men in the National Health and Nutrition Examination Survey [32], and has been used as a measure of weight misperception in several published reports [26, 27]. Because few women in our sample had extreme weight perceptions or perceived themselves as underweight, we collapsed response categories into two groups: overweight and average/underweight.

Using BMI and perceived body weight status, we created two body weight misperception categories: normal weight women who identified themselves as overweight ('overassessors'), and overweight/obese women who identified themselves as average or underweight ('underassessors'). Women who correctly perceived their body weight status were classified as either normal weight or overweight/obese accurate assessors, and were also included in our analysis.

We used prenatal medical records to obtain pregnancy weights, and calculated total gestational weight gain as the difference between the last clinically recorded weight before delivery and self-reported pre-pregnancy weight. Based on current guidelines by the Institute of Medicine published in 1990 [11], we classified gestational weight gain as inadequate, adequate, or excessive. These guidelines recommend that women with a normal pre-pregnancy BMI (19.8–26.0 kg/m²) should gain 11.5–16 kg, that overweight women (BMI of 26.1–29.0 kg/m²) should gain 7.0–11.5 kg, and that obese women (BMI greater than 29.0 kg/m²) should gain at least 7.0 kg. As we combined overweight and obese women in our analysis, we also set an upper limit of 11.5 kg for these heaviest women [9, 14]. Weight gain above recommended ranges is considered excessive. In this paper, we defined our main outcome as excessive gestational weight gain (vs. adequate or inadequate), as women who over-gain are at greatest risk for obstetric complications and postpartum obesity [1, 3‐10].

We collected information on maternal race/ethnicity, age, education, parity, employment, household income, and smoking habits, all reported by the women at their first study visit. In mid-pregnancy (approximately 26–28 weeks gestation), participants reported information about their leisure-time physical activity and diet. We were interested in evaluating mid-pregnancy behaviors as potential intermediates in the pathway between body weight misperception and excessive gain in pregnancy. Vigorous activity was of primary interest as a measure of physical activity because in this cohort we observed an association between less time spent doing vigorous activity and excessive gestational weight gain [22]. We asked each participant, "In the past 3 months, on average, how many hours per week have you spent engaged in vigorous recreational activities or sports such as jogging, swimming, cycling, aerobic dance, skiing, or other similar activities?" Our choice of examples of activities was influenced by the Physical Activity Scale for the Elderly [33], the Paffenbarger physical activity questionnaire [34], and knowledge of activities common to women in the northeastern US. Dietary intake was assessed via the semi-quantitative food frequency questionnaire (FFQ), slightly modified for use in pregnancy from the extensively validated FFQ used in the Nurses' Health Study [35, 36]. We queried participants about their dietary intakes during the preceding 3 months. A priori, we felt it reasonable to focus on intake of fried foods away from home, fruit and vegetable consumption, and intake of a vegetarian diet [22]. We also collected data about a history of depressive symptoms prior to pregnancy with the question, "Before this pregnancy, was there ever a period of time when you were feeling depressed or down or when you lost interest in pleasurable activities most of the day, nearly every day, for at least 2 weeks," along with an indication that a professional had previously diagnosed or treated the participant for depression. We calculated gestation length from the last menstrual period, or from the second trimester ultrasound if the two estimates differed by > 10 days.

Because some clinicians may use BMI categories based on WHO guidelines in practice [37, 38], we performed a secondary analysis using current WHO recommendations for normal weight (BMI 18.5–24.9 kg/m²) and overweight/obese (BMI > = 25 kg/m²) to classify both misperception of body weight and categories of gestational weight gain. The results were essentially unchanged from the primary analysis (data not shown).

Given the high prevalence of excessive gestational weight gain, odds ratios (OR) are poor estimators of relative risks. For ease of interpretability, in addition to ORs and 95% confidence intervals (CI), we present risks calculated from multivariable-adjusted predicted prevalences for representative subgroups. We did this calculation by choosing covariate values corresponding to groups of interest and inverting the logit, thus back-transforming the individual predicted logits to obtain predicted probabilities [39, 40]. We used SAS version 9.1 (SAS Institute, Cary, NC) to carry out all analyses.