Researchers have presented further evidence of how maternal characteristics can influence the development of children born after in vitro fertilisation (IVF). A study of all 8941 IVF children born in Sweden between 2002 and 2006 where only one baby was born as a result of a single pregnancy showed that maternal age, primiparity (first birth), smoking, maternal infertility and body mass index (BMI), both over and underweight, were associated with an adverse perinatal outcome.