On average, each mother gained 14.8 kg (SD 5.2) during her pregnancy, with an average of 0.4 kg per week (range: 0.0 to 0.9; SD 0.1) weight gain. Of the 6632 participants 1666 (25%) gained inadequate, 2571(39%) adequate and 2349 (36%) excessive weight during pregnancy according to IOM categories. 655 (9.9%) participants were underweight, 4924 (74.3%) had healthy weight, 778 (11.7%) were overweight and 275 (4.2%) were obese according to their pre-pregnancy BMI. 5079 (77.7%) mothers had normal delivery, 771 (11.8%) had caesarean delivery and the rest 678 (10.4%) had other deliveries including low forceps, mid forceps, ventouse, assisted breech, trial forceps and combined methods. Only 269 (4%) births delivered as premature and 582 (8.9%) had pregnancy complication. Mean birth weight was 3384.2 (SD 516.1) gm and placenta weight was 602.2 (SD131.5) gm. On average women stayed 4.3 (SD 1.6) days in the hospital from delivery to discharge. For normal vaginal delivery the mean length of hospital stay was 4.00 (SD 1.33) days, for caesarean delivery 6.21 (SD 1.58) days and for other types of delivery it was 4.80 (SD 1.55) days.
The unadjusted association of maternal characteristics with IOM categories are presented in additional file 1 table S2. Mothers with lower educational attainment, those of Aboriginal-Islander origin, and those who never smoked and abstained from alcohol prior to pregnancy were more likely than other women to gain excessive weight during pregnancy. Mothers who gained excessive weight were more likely to have experienced pregnancy complications, have had their infant delivered by caesarean section and to have had higher birth weight infants.
Table 1 shows odds of being caesarean and other deliveries compared to normal deliveries, preterm births compared to normal birth and high risk compared to normal risk pregnancy by IOM, pre-pregnancy BMI categories and gestational weight gain per 0.1 kg/week. The results are presented for the 6528 (91%) mothers with complete data on all variables included in the fully adjusted model. Model 1 shows maternal age adjusted odds ratios (OR) and model 2 shows OR adjusted for all other covariates. In the age adjusted model, mothers who gained excess weight during pregnancy were 1.29 (95% CI: 1.09, 1.54) times as likely to go for caesarean delivery compared to mothers who maintained healthy weight gain during pregnancy. Similarly, overweight mothers were 1.40 (1.13, 1.75) times and obese mother were 2.19 (1.61, 2.98) times as likely to experience caesarean delivery compared to their counterpart. For 0.1 kg/week increase of GWG, each mother was at 9% higher risk to experience caesarean delivery. Mothers who gained inadequate weight or were underweight before pregnancy, were at greater risk of delivering preterm and were at less risk if they gained excess weight. In contrast, those mothers who gained inadequate weight were at a decreased risk of developing pregnancy complications and those who gained excess weight were at greater risk of experiencing pregnancy complications. Similarly, for 0.1 kg/week increase of GWG the risk of preterm births was low, however, the risk was greater for experiencing pregnancy complications. All the association remain consistent adjusting for potential confounding factors.
Those mothers who did not gain adequate weight during pregnancy delivered a 190.63 (-221.05,-160.20) gm lighter baby and those who gained excess weight delivered a 206.45 (178.82, 234.08) gm heavier baby compared to mother who gained healthy weight (table 2). Similarly, mothers who were underweight before pregnancy delivered a lighter baby and overweight mothers delivered a heavier baby. For 0.1 kg/week increase of GWG, each mother delivered a 81.52 gm heavier baby. Similarly, for placenta weight those mothers who were underweight or did not gain adequate weight had lower placental weight, and conversely, it was greater if they gained excess weight or were overweight or obese.
Additional file 1, table S3 shows unadjusted mean length of hospital stay postnatally by pre-pregnancy BMI categories and IOM categories of weight gain. Excess pre-pregnancy BMI was associated with excess length of postnatal stay in hospital. These unadjusted results also show that excess weight gain in pregnancy is associated with excess length of postnatal stay.
The multivariable analyses for the association of pre-pregnancy BMI categories with length of postnatal hospital stay is presented in Table 3. In the age adjusted model (i.e. model 1), on average an obese mothers stayed 0.30 (0.10, 0.49) days longer in hospital postnatally compared to mothers with a healthy BMI. However, this association was attenuated with adjustment for potential confounders or mediators, particularly by adjustment for pregnancy complications and caesarean delivery.
Table 4 shows the multivariable analyses for the association of IOM categories of weight gain with length of postnatal hospital stay. Women who gained excessive weight during pregnancy (0.19 kg per day on average) stayed on average longer in hospital compared with those who gained adequate weight. These differences remained robust after adjustment for confounding factors (model 2) but were attenuated (approximately 20%) by adjustment for potential mediators (model 3). Since IOM categories combine pre-pregnancy BMI with weight gain categories no additional adjustment for pre-pregnancy BMI was made in these analyses.
Additional file 1, figure S1 shows mean differences in length of hospital stay from delivery to discharge by maternal weight gain per week per 0.1 kg/week during pregnancy. In the age adjusted model, a 0.1 kg/week excess maternal gestational weight gain was associated with 0.09 of a day (95% CI: 0.06, 0.11) longer hospital postnatal stay on average. There was a marginal attenuation after adjustments were made for the mediating effect of pregnancy complications (model 4).
In additional analyses, when we further adjusted the association of IOM or GWG with hospital stays by parity, maternal depression, family income and placenta weight, the associations remained unchanged (results available from author on request).