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Thesis Format



Master of Science


Epidemiology and Biostatistics


Campbell, M. Karen

2nd Supervisor

Klar, Neil

Joint Supervisor


Maternal obesity and multiple births have adverse effects on fetal growth; prevalence of both are increasing in Canada. We explored associations between maternal obesity and infant size for gestational age using data from a London perinatal database. Birthweight for gestational age was assessed using standards published by Robertson (2002) to classify Small for Gestational Age (SGA) and Large for Gestational Age (LGA) in 30396 singletons and 1346 twins. Associations were estimated using logistic regression for singletons and the GEE extension of logistic regression for twins. Increased maternal pre-pregnancy BMI was statistically significantly associated with decreased odds of SGA in singletons (p< 0.0001), and increased odds of LGA in singletons and twins (p< 0.0001, p= 0.0004 respectively). Results suggest that maternal BMI may influence size for gestational age differently in singleton and twins.

Summary for Lay Audience

The St. Josephs’ Health Care and London Health Sciences Centre Perinatal Database contains valuable information on pregnant mothers and their newborns.

We used this database to explore mothers’ body mass index (BMI), and how this affected growth of their babies. While this has been studied in singleton babies (babies that are born following a pregnancy where the mother carried only a single baby), it has not been studied in babies who are twins.

As an additional objective, we wanted to know if mothers’ BMI affected the growth of singleton and twin babies in similar ways. A twin pregnancy can be more complicated for the mother, the babies, and the doctors taking care of them, compared to a singleton pregnancy. For this reason, twins are often excluded from research studies. We wanted to compare singleton and twin pregnancies because many studies exclude twins from their research.

We focused on mothers’ BMI because this can be related to other medical conditions that can harm the pregnancy. To study growth, we used a measure known as “Size for Gestational Age, which assesses the newborn’s weight at birth, relative to the weight we expect for a baby born from a pregnancy of this length (gestational age). This measure allows us to better compare newborn growth. Newborns with below-normal rates of growth are “Small for Gestational Age (SGA)” while newborns with above-normal rates of growth are “Large for Gestational Age (LGA)” compared to the population. Identifying SGA and LGA newborns can help doctors recognize newborns that might need further health care.

We found that heavier mothers were more likely to have a bigger, or LGA singleton or twin baby, and less likely to have a smaller, or SGA singleton baby. However, it is important to remember that some heavier mothers can still have smaller babies, and that some thinner mothers can still have larger babies. This was previously known for singletons, but we found this also to be true for twins. Our results were unable to determine conclusively whether this may just be a random finding because the number of twins in our study was small. Therefore. further study is required. These results are important, because they can help doctors better take care of mothers of all sizes with singleton and twin pregnancies.