They assume it's an environmental factor but I wouldn't be so sure. It's certainly possible that this bias is more related to parents behaviour. Some parents w

They assume it's an environmental factor but I wouldn't be so sure. It's certainly possible that this bias is more related to parents behaviour. Some parents would conceive in August resulting in their children being born in May.... Other parents would conceive in January resulting in children being born in October. People which conceive during different seasons might be different both genetically and behaviourally which could influence offspring's health by means of genetic factors but also habits learned from their parents during childhood (dietary choices for example). Or not.