Design of the SNiP-I study
The present study is part of the population-based birth cohort study `Survey of Neonates in Pomerania´ (SNiP-I). The design of the SNiP-I study has been described in detail by Ebner et al . In short, the SNiP-I study was conducted from February 2002 to November 2008 in the region of Pomerania in Northeastern Germany. More than 270 variables covering personal data, medical records and socioeconomic background were collected and recorded from each participating mother-child dyad. From all non-participants, excluded individuals and non-responders, a minimum dataset was compiled comprising data on the health status of women and their newborns but lacking detailed information about environmental parameters.
The baseline of the SNiP-I birth cohort includes data on 5,800 mother-child-dyads. For the purpose of this analysis, only singleton pregnancies with life or stillbirth and with known maternal pre pregnancy BMI were included. This subpopulation of the baseline SNiP-I birth cohort comprised 4,667 mother-child-dyads (Figure 1).
Pre-pregnancy Body-Mass-Index (BMI)
Women were categorised into different BMI groups according to the classification recommended by WHO . Height (in cm) and pre-pregnancy body mass (in kg) were reported by women using a standardised self-administrated questionnaire during the stay at the obstetrical ward. The content of this questionnaire was described by Ebner et al in detail .
Selected maternal pre-pregnancy diseases as defined by Kersten et al were stratified by body mass index .
The stratification pattern for educational level followed the already published pattern . Persons without a school diploma, being still at school or with five years, or less, of secondary school were pooled together and were referred to as having a low educational level. Persons with 6 years of secondary school (German `Realschulabschluss´) were included in the second level, referred to as the middle educational level. The third level included persons with 8 years of secondary school (German `Fachhochschulreife´ or `Abitur´) and was referred to as the mid-high educational level. The highest educational level was assigned to persons who graduated and was referred to as a high educational level.
Definitions of smoking and alcohol use
In this paper, we did not analyse the dose effect of tobacco and alcohol consumption on pregnancy and neonatal outcomes. Therefore, we did not differentiate the cohort by the amount of alcohol consumed or tobacco smoked. Instead, we used a simple dichotomous classification: `smoker/non-smoker´ and `drinker/non-drinker´. A woman was classified as a smoker if she declared to smoke during the last four weeks before delivery. Similarly, a woman was classified into the group of drinkers if she continued to drink alcohol during pregnancy, irrespective of the amount and time period of consumption.
Definition of small-for-gestational-age and large-for-gestational age
Small-for-gestational-age (SGA) was defined as birthweight below the 10th percentile for their estimated gestational age. Large-for-gestational-age (LGA) was defined as a birth weight greater than the 90th percentile adjusted for gestational age .
The gestational age was based upon the date of the mother's last menstrual period according to records in maternity card.
Diagnosis of neonatal hypoglycaemia
Neonatal hypoglycaemia was diagnosed using biochemical parameters according to national guidelines. A plasma glucose concentration of 45 mg/ml (2.5 mmol/l) within the first 24 hours after birth was used to diagnose neonatal hypoglycaemia. It was a routine policy to screen babies of mothers with gestational diabetes, preterm babies, babies with low birth weight <2,500g, as well as all SGA- and LGA-babies for hypoglycaemia.
Definition of neonatal asphyxia
Neonatal (birth) asphyxia was defined according to medical diagnosis and included all ICD-10 codes P21.-, which included asphyxia of any grade (severe asphyxia, P21.0, mild and moderate birth asphyxia, P21.1, as well as unspecified birth asphyxia, P21.9). Medical diagnosis was taken from child’s medical records.
Conditions for admission to neonatal care
According to the institutional policy and the national guideline, the personnel attach great importance to maintaining mother-child contact even in the case of pathology. Babies and their mothers were left at the maternity ward as long as the conditions allowed such a situation. The national guideline clearly defines when the neonate should be transferred to the neonatal ward, particularly when intravenous administration of glucose was necessary or in case of severe symptoms. For the purposes of the study, `admission to neonatal care´ included both neonatal intensive care and special care with respect to the newborn’s condition and needs.
Definition of monthly available equivalent income
The need for housing space, electricity, and other essentials does not increase proportionally with the higher number of members in the household. To account for this phenomenon, we have used equivalence scales, based on the OECD-modified scale [16, 18].
Potential mediators and confounders
We have considered the following factors as potential mediators in the pathway between maternal low BMI before pregnancy and adverse pregnancy and neonatal outcomes: tobacco smoking and alcohol consumption during pregnancy, maternal age, available monthly equivalent income, and parity. These variables were assessed by the self-administered questionnaire. Ethnicity is another potential confounder; however, this factor could not be analysed because less than 2% of the population were not Caucasian.
All data were stored using a Microsoft Access 2002 (Microsoft Corporation, Redmond, WA, USA) database.
Continuous data are reported as the medians with the 25th and 75th percentiles; categorical data are expressed as the absolute numbers and/or percentages. Wilcoxon test and two-tailed χ2 test were used to calculate p-values for continuous and categorical variables, respectively. First, associations of socioeconomic status (educational level, income, employment status), and adverse habits (smoking and alcohol consumption) with mothers’ low pre-pregnancy BMI were analysed by linear, logistic and multinomial logistic regressions adjusted for confounders. Second, associations of mothers’ low pre-pregnancy BMI with adverse pregnancy and neonatal outcomes, such as birth weight, gestational age, hypoglycaemia, admission to the neonatal care unit, mode of delivery were analysed. The respective confounders used in the multivariate analyses are mentioned in the legends of each table. In all analyses, p < 0.05 was considered statistically significant. All statistical analyses were carried out using Stata 16.0 (Stata Corporation, College Station, TX, USA).