Methods
Study design, study population
This was a multisite cohort study in Western Sydney with enrolled women
from four participating hospitals.
Pregnant women aged 18 years and over who had GDM in their first
pregnancy and had an expected date of delivery from 1st March 2017 to
30th April 2019 were recruited. Recruitment occurred from 20 weeks’
gestation. Exclusion criteria included pre-existing diabetes, pre-term
first pregnancy (<37 weeks), multiple birth first pregnancy, a
history of more than three miscarriages, fetal anomaly or stillbirth.
Women who had care in their first pregnancy in a country other than
Australia were also excluded from the study.
All potentially eligible women were identified through the electronic
maternity database from information provided at their booking visit.
Eligible women were approached in the antenatal clinic or in hospital
during the first two days postpartum at the four participating
hospitals.
At participating hospitals, GDM was diagnosed on the basis of the 1998
Australasian Diabetes in Pregnancy Society criteria.19This involves a 75g oral glucose tolerance test (OGTT) with a fasting
blood glucose level (BGL) of ≥5.5 mmol/L and/or a 2 h BGL ≥8.0 mmol/L.
The OGTT at 1 h was also collected and results made available to
clinicians, however, it is not routinely used for diagnostic purposes.
All pregnant women were tested for GDM at 24–28 weeks’ gestation, and
earlier if clinically indicated. Management of GDM with insulin or
metformin was defined as medical treatment inclusive of dietary
modifications. Standard treatment for GDM includes blood glucose
monitoring, dietician review, insulin treatment if glucose targets are
not met. All women are advised to maintain a similar healthy diet after
the birth of their baby and have follow-up with their family doctor.
Questionnaire
An expert panel, including lactation consultants (International Board
Certified Lactation Consultants, IBCLC), endocrinologists and a maternal
fetal medicine specialist, designed a task-specific, four-page health
and lactation questionnaire with breastfeeding intensity assessment
(Supplemental S1). Questions primarily related to the participants’
previous pregnancy and were based on maternal recall. Co-design input
was through a consumer trial of questionnaires, with revision from
feedback, to a final questionnaire design. After revision, 92% (11/12)
of consumers agreed or strongly agreed that the questions were easy to
understand and ‘accurately records how I fed my baby’.
Length of breastfeeding was assessed by a question on baby’s age when
all breastfeeding stopped. The Breastfeeding Length Intensity Scoring
System (BLISS) was used to assess breastfeeding intensity in the first
three months postpartum. The level of intensity is a determination of
the amount of maternal breastfeeding inclusive of expressing, in
relation to formula feeding. The highest intensity is mostly or
exclusively breastfeeding, and the lowest intensity is mostly formula
feeding. The scoring of intensity and determination of four levels of
intensity was validated by two consultants (IBCLC) who assessed typical
optimal feeding patterns and correlated the BLISS score to length of
breastfeeding with a maximum score of 25. Assessment of infant feeding
by BLISS occurred via maternal recall for the time periods as follows;
the first week post-partum (score/6), during the first month (score/7),
one to three months (score/6) and at three months (score/6).
Breastfeeding only at any time point was given full marks, formula only
feeding scored nil, and scores were incrementally reduced for 1–4
formula feeds a week, 1–3 per day, with four or more formula feeds per
day scoring similar or equal to full formula feeding (Supplemental S1).
High intensity feeding was classified as a score between 19–25,
mild-moderate intensity 7–18, and a low-intensity score was defined as
between 0–6.
Data Collection
Participating women completed the self-administered questionnaire. Women
who utilised an interpreter for their obstetric appointment were
verbally given the questionnaire with the assistance of the interpreter.
Demographic information collected included self-identification of
ethnicity, country of birth and length of time living in Australia if
they were a migrant. Second pregnancy details were collected from the
participating hospitals’ electronic records and included booking BMI,
diagnosis of GDM and pregnancy OGTT results.
Statistical
Analysis
Demographic and clinical variables were summarised between outcome
groups, with percentages for categorical variables and median and
interquartile range (IQR) for continuous variables, as all those
included were not normally distributed. The relationships between length
of breastfeeding and the BLISS score with the OGTT results from the
subsequent pregnancy were analysed at each OGTT measurement time
separately using linear regression. This was done after the relationship
between both length of breastfeeding and BLISS score and the OGTT
measurement was found to be different for each of the 3 OGTT
measurements (interaction) were found to be different using a general
linear model to account for the correlation within a woman. All
relationships with the diagnosis of GDM were assessed with a general
linear model with a log link and the results summarised as relative
risks. Both length of breastfeeding and BLISS score were dichotomised
for these models due to the relationships using the continuous variable
not being linear. Both the models for OGTT and for diagnosis of GDM were
fitted unadjusted and then adjusted for medical treatment of GDM in the
first pregnancy, BMI as ≥30 kg/m2 or <30
kg/m2, age at current pregnancy ≥30 or <30
years old, and ethnicity as self-identified Caucasian, South Asian or
other. Family history of diabetes was assessed but did not contribute to
any model so was excluded. There was no imputation of missing data and
no adjustment for multiple comparisons. All analyses were done with SAS
9.4 SAS Institute Inc., Cary, NC, USA.