Gestational weight gain

After all babies were born, the weight and gestational age data for each prenatal visit were obtained in the pregnant women’s health records. Based on these data, the average GWG per week in the second and third gestational trimesters was calculated using the method established by the Institute of Medicine⁽³¹⁾ (i.e., difference between the last and the first weight measured in each trimester of pregnancy (second and third), divided by the number of gestational week in this interval). The average weekly GWG in each trimester was expressed in g/week.

Just to describe the sample, we also classify the weekly GWG as insufficient, adequate and excessive according to the same reference⁽³¹⁾, but considering GWG during second and third trimesters together, what is more usual in the literature^(26,31,32). This evaluation required the calculation of each patient’s pre-gestational BMI and their classification into underweight, normal weight, overweight and obese⁽³³⁾. To this end, weight before pregnancy and height were also obtained from the medical records.

In order to avoid errors in this classification due to under- or overestimation of pre-gestational weight, the pre-gestational weight recorded in the medical records (usually self-reported) was compared with the first weight measured in the first gestational trimester, the latter being considered when the difference was >2 pounds^(34,35).

The option to take average weekly GWG in the second and in the third trimesters of gestation as the outcome was due to the fact that this period, which goes from the 14th to the 42nd gestational week, is recognised as the phase of greatest maternal and newborn weight gain and which most influences their health⁽³¹⁾. The alternative – assessing total gestational weight gain – was not considered because it is not usual in Brazilian maternity hospitals to weigh pregnant patients upon arrival for hospitalisation for childbirth. Thus, we did not have information about the patients’ final weights.