... effect of birth order isalreadypresentatthetimeofbirthofthechild,then the effect of hygiene later in childhood may not be asrelevant as is currently thought. In support of this expla-nat ion, othe ... investigation, the hygiene hypothesis may explain observed associationsbetween family size, birth order, day-care attendance,and the risk of asthma and allergy. According to the hygiene concept, the effect ... the increasing prevalence of atopy in childhood.Previous studies have assessed two related ideas - the hygiene hypothesis and the birth order effect [9]. The hygiene hypothesis suggests that exposure...