OBJECTIVE: To investigate the duration of the presence of maternal antibodies to measles in infants. DESIGN: Prospective study (May 2006 to November 2008). SETTING: Five hospitals in the Province of Antwerp, Belgium. PARTICIPANTS: Of 221 pregnant women recruited, 207 healthy woman-infant pairs were included-divided into a vaccinated group (n=87) and naturally immune group (n=120), according to vaccination documents and history. MAIN OUTCOME MEASURE: Measles IgG antibodies measured by enzyme linked immunosorbent assay (ELISA) at seven time points (week 36 of pregnancy, birth (cord), and 1, 6, 9, and 12 months); decay of maternal antibody in infants modelled with linear mixed models. RESULTS: Vaccinated women had significantly fewer IgG antibodies (geometric mean titre 779 (95% confidence interval 581 to 1045) mIU/ml) than did naturally immune women (2687 (2126 to 3373) mIU/ml) (P<0.001). Maternal values were highly correlated with neonatal values (r=0.93 at birth). Infants of vaccinated women had significantly lower antibody concentrations than did infants of naturally immune women (P<0.001 at all ages over the follow-up period). Presence of maternal antibodies endured for a median of 2.61 months-3.78 months for infants of naturally infected women and 0.97 months for infants of vaccinated women. At 6 months of age, more than 99% of infants of vaccinated women and 95% of infants of naturally immune women had lost maternal antibodies according to the model. CONCLUSIONS: This study describes a very early susceptibility to measles in infants of both vaccinated women and women with naturally acquired immunity. This finding is important in view of recent outbreaks and is an argument for timeliness of the first dose of a measles vaccine and vaccination of travelling or migrating children under the age of 1 year