Design and selection of the study participants
This is a cross-sectional observational study aimed at women who have been mothers between 2013 and 2018 in Spain. This study has received the approval of the Clinical Research Ethics Committee of Alcázar de San Juan, in Spain, with protocol number 92-C.
The participants were women over the age of 18, who understood Spanish and who agreed to fill in the questionnaire. Before filling in the questionnaire, the participants read the information sheet about the purpose of the study and give their consent to participate in it by ticking a box. After this, they were provided with the necessary information to be able to complete the questionnaire. Participants could voluntarily provide an email address or phone number through which they would be contacted in case any additional information related to the study was needed.
For the sample size estimation, the maximum modelling criterion that requires 10 events for each independent variable to be included in the multivariate model has been used [15]. For this sample calculation, the prevalence of delayed lactogenesis II has been used as the main event, which takes place in women with obesity around 58% [9]. Considering a minimum of 15 independent variables, a minimum of 150 women with delayed lactogenesis II would be required, representing a minimum total population of 259 women under study within this scenario. However, the research team decided to include all women who met the inclusion criteria in order to obtain estimates of greater precision.
For data collection, an anonymous online questionnaire was designed with 54 items [47 yes/no questions and 7 multiple answer questions) on sociodemographic and obstetric variables, and on complications during pregnancy, delivery, and postpartum (Additional file 1). The questionnaire was disseminated among the Spanish lactation and postpartum support associations and groups, and those responsible for these groups were in charge of disseminating them among their members (Additional file 2).
The variables included in the study were:
The main independent variable was self-reported Body Mass Index prior to gestation, categorised normoweight (18.5–24.9), overweight (25–29.9), obesity grade I (30.0–34.9), obesity grade II (35.0–39.9), and obesity grade III (≥ 40.0).
The independent adjustment variables were maternal age, income level, level of education, tobacco consumption, number of children, number of births, gestational age at the time of delivery, hypertension during pregnancy (high blood pressure during pregnancy), diet-controlled gestational diabetes (high sugar levels in blood during pregnancy, controlled with diet), insulin-controlled gestational diabetes (high sugar levels in blood during pregnancy, controlled with insulin), hyperthyroidism (high thyroid hormone levels), hypothyroidism (low thyroid hormone levels), anaemia (low iron levels in blood), intrahepatic cholestasis (reduction or obstruction of flow of bile in the liver), risk of preterm birth (risk of giving birth before week 37), deep vein thrombosis, oligohydramnios (decrease in amniotic fluid levels), polyhydramnios (increase in amniotic fluid levels), altered foetal heart rate (FHR) during delivery, stained amniotic fluid (AF), vaginal bleeding, non-progression of delivery, cephalopelvic disproportion, intrapartum fever, intrapartum preeclampsia, use of epidural analgesia, use of general anaesthesia, induced delivery, end of delivery, episiotomy, tearing, low birthweight (2.500 g), macrosomia (> 4.000 g), newborn admission, postpartum-related subsequent maternal surgery, maternal admission into intensive care unit, and maternal re-admission. The women were asked to mark the existence of a pathology only after having been diagnosed by a physician.
In addition, 3 “composite” morbidity variables were created: pregnancy (including Hypertension during pregnancy, diet-controlled gestational diabetes, insulin-controlled gestational diabetes, hyperthyroidism, hypothyroidism, anaemia, intrahepatic cholestasis, risk of preterm birth, deep vein thrombosis); delivery (including altered foetal heart rate (FHR) during delivery, stained amniotic fluid (AF), vaginal bleeding, non-progression of delivery, cephalopelvic disproportion, intrapartum fever (body temperature higher than 38 °C), intrapartum preeclampsia, induced delivery, end of delivery by caesarean, and severe tearing (type III-IV); neonatal (including prematurity, low birthweight (< 2.500 g), macrosomia (> 4.000 g), and neonatal admission to intensive care unit; and postpartum (including maternal postpartum surgery related to the delivery, maternal admission to intensive care unit, and maternal re-admission).
The dependent variables were: complications during lactation (starting skin-to-skin contact, starting breastfeeding in the first hour, and exclusive breastfeeding to hospital discharge), and complications or discomfort associated with breastfeeding during hospitalisation and at home.
Statistical analysis
First, a descriptive analysis was performed using absolute and relative frequencies for the categorical variables. A bivariate analysis was then performed including the main sociodemographic, obstetric, neonatal, lactation and BMI characteristics at the time of delivery; this was done by using the Pearson’s chi-squared test with lineal trend. The analysis was performed to detect potentially confounding variables between the main dependent variables related to breastfeeding and BMI. In the next step, a multivariate analysis was carried out using binary logistic regression (through the ENTER procedure), where all the variables that could potentially be related to BMI were incorporated. Then, the Crude Odds Ratios (OR) and Adjusted Odds Ratios (AOR) were calculated with their respective 95% confidence intervals (95% CI) to determine the relationship between BMI and complications during breastfeeding (starting skin-to-skin contact, starting breastfeeding in the first hour, and exclusive breastfeeding to hospital discharge) and complications or discomfort both during hospitalisation and at home. All analyses were performed using the SPSS v24.0 statistical package.