First-born children are more likely to suffer from allergies than their younger brothers and sisters, Japanese scientists have found.
The researchers discovered that multiple births build up the immune system in the womb which is transferred to babies.
It means second and third children are less likely to suffer from hay fever or develop food allergies.
Scientists surveyed the parents of 13,000 school children aged seven to 15, and asked them the order of their children and what allergies they had.
The findings showed four per cent of first-born children had rhinitis, conjunctivitis and food allergies compared with 3.5 per cent of second-born children.
Meanwhile, just 2.6 per cent of third-born children suffered from allergies.
"It has been established that individuals with increased birth order have a smaller risk of allergy," the Daily Mail quoted Dr Takashi Kusunoki, who led the study for the Shiga Medical Center for Children and Kyoto University in Japan, as saying.
"However, the significance of the effect may differ by allergic diseases.
"Further evaluation should shed light on the role of pre and post-natal circumstances on the development of childhood allergy," Kusunoki added.