Neonatal antibiotic exposure linked to reduced weight and height in boys Exposure to antibiotics in the first days of life is thought to affect various physiological aspects of neonatal development. A new study, led by Bar-Ilan University's Azrieli Faculty of Medicine, reveals that antibiotic treatment within 14 days of birth is associated with reduced weight and height in boys - but not girls -- up to the age of six. By contrast, the study showed significantly higher body mass index (BMI) in both boys and girls following antibiotic use after the neonatal period, and within the first six years of life.