Rates of birth defects linked to twins and triplets have almost doubled since the 1980s, research has shown.
The trend may partly be explained by the growth of IVF treatment, which can lead to multiple births.
Scientists studied data on 5.4 million births in Europe from 1984 to 2007, of which 3% involved multiple deliveries.
During this time, the prevalence of congenital abnormalities arising from multiple births increased from 5.9 per 10,000 births to 10.7.
On average, the risk of birth defects was 27% higher for multiple than for singleton births. This risk increased over time.
Study co-leader Professor Helen Dolk, from the University of Ulster, said: "The co-occurrence of multiple birth and congenital anomaly among live-borns places particular demands on parents and health services. This may be even more relevant for the one in nine affected twin pairs where both babies have a congenital anomaly.
"The increase in multiple birth rates may be explained by changes in maternal age and increased use of ART (Assisted Reproductive Technology). It is clear that more research needs to be done to determine the contribution of ART to the risk of congenital anomalies in multiple births."
The research is published in BJOG: An International Journal of Obstetrics and Gynaecology.
Transferring a number of embryos into a woman's womb during IVF treatment is known to increase the risk of multiple births. Fertility clinics are now being advised to adopt a single embryo transfer (SET) policy.
John Thorp, deputy editor-in-chief of BJOG, said: "This increase in babies who are both from a multiple pregnancy and affected by a congenital anomaly has implications for pre and post-natal service provision. Extra specialised help should be put in place for affected families, recognising that there are now nearly double as many affected families than there were 20 years ago."