Study design
This study was a retrospective analysis of data from the BiB cohort.
White British and South Asian pregnant women were the populations of
interest. Women from other ethnic groups, or where ethnicity data were
missing, were excluded from the analysis. The two outcome variables were
antenatal depression and anxiety. In the BiB cohort, mental health was
assessed using the GHQ. The GHQ is a validated self-report questionnaire
with 28 items relating to the respondent’s current mental state, ability
to carry out functions and daily activities and appearance of new and
distressing phenomena.13 The 28 items are grouped into
four categories each used to identify symptoms of certain psychiatric
disorders. Since this study focused on depression and anxiety as the
most common mental health conditions during pregnancy, the total Likert
score of the subscales for depression and anxiety (D, items 22-28; B,
items 8-14) were used in the analysis (Appendix S1).13There are no agreed thresholds for the subscales to indicate depression
or anxiety, but studies suggest that the cut-off score should be based
on the mean/median of the sample of interest.14,15 In
this study, the median was used due to the non-normality of
distribution. This was 0 for depression and 6 for anxiety. Therefore, a
score of >0 was used to indicate depression and
>6 indicated anxiety.
The main exposure variable was maternal early pregnancy BMI. Data from
the BiB cohort includes information on the mother’s booking BMI
calculated using measured height and weight between 10-12 weeks of
pregnancy.16 A realistic lower limit of 11
kg/m2 was set as this has been shown to be the lowest
BMI for survival in women.17 An upper limit of 80
kg/m2 was based on the frequency distribution in the
data from the BiB cohort and a published study.18Women with a booking BMI outside this range were excluded from analysis
(n=720, 6.5%). BMI was analysed as a categorical variable due to the
inclusion of underweight which is also associated with increased
depression and anxiety,19 therefore, a continuous
analysis may skew the results. BMI was stratified by the WHO’s
classification. For White British women, the categories were:
underweight, <18.5 kg/m2; recommended
weight, 18.5-24.9 kg/m2; overweight, 25-29.9
kg/m2; and obese class 1, 30.0-34.9
kg/m2; class 2, 35.0-39.9 kg/m2;
class 3, ≥40.0 kg/m2.2 The
categories used for South Asian women were: underweight, <18.5
kg/m2; recommended weight, 18.5-22.9
kg/m2; overweight, 23-27.49 kg/m2;
and obese class 1, 27.5-32.49 kg/m2; class 2,
32.5-37.49 kg/m2; class 3, ≥37.5
kg/m2 .20 A secondary analysis was
performed using the general population BMI criteria (Table S1)for South Asian women due to the current lack of guidance in the UK for
using Asian-specific criteria in pregnancy.
Additional variables included in the adjusted models were maternal age,
maternal education, area of residence deprivation (based on postcode)
and maternal smoking. Maternal age (years) was analysed as a continuous
variable. Maternal education was defined as mother’s highest educational
qualification (equivalised) with the following categories: none, GSCE
equivalent, A-level equivalent and higher than A-level (used as
reference group). The Index of Multiple Deprivation (IMD) was used to
categorise area of residence deprivation. The IMD is a measure of
relative deprivation for small areas in England and is the most widely
used tool to measure deprivation in health-related research in the
UK.21 Since BiB was carried out in Bradford, the
national IMD quintiles would be of limited use for this study because
Bradford has a higher level of deprivation compared to most areas of the
UK.12 Therefore, the deprivation data were skewed
towards the most deprived quintiles for this population. A binary
variable was created with quintiles 2-5 combined to represent lower
levels of deprivation (used as the reference group) and quintile 1
represented the highest level of deprivation. Maternal smoking during
pregnancy was a binary variable (yes/no).