| Summary: | Twins often show signs of physical and cognitive developmental delays compared with singletons at school age, but tend to catch up by young adulthood. Many previous studies focused on early childhood and beyond, and twins with health issues. In this thesis, I investigated the infancy period of healthy twins in the Birmingham Registry for Twin and Heritability Studies. More specifically, I explored twins’ developmental trajectories compared with singleton standards; ante-, peri-, and postnatal factors related to developmental skills; and the association of maternal occupation with twin pregnancy outcomes. Additionally, I studied the heritability of body mass index (BMI) over a lifespan in a meta-analysis of twin studies. Twins had worse developmental skills and were small for their age compared with singleton standards. Birth weight was not strongly associated with developmental skills as in previous studies; however, larger antenatal head circumference was negatively associated with postnatal development. Higher occupational psychological strain was related to shorter gestations. Finally, heritability of BMI remained high over a lifespan. Results were discussed in light of current clinical and health and safety guidelines. Suggestions for further research and dissemination of findings to parents of multiples were also discussed in the final chapter of this thesis.
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