Effect of Month of Birth on Mean Birth Length in Austrian Newborns Born Between 1984 and 2021

The length of newborns shows seasonal fluctuations, but the patterns of these fluctuations vary greatly. This study analyses the fluctuation in birth length by birth month and temporal changes in Austrian newborns from the 1984 to 2021 birth cohorts.

A total of 2 317 927 singleton-term births between 1984 and 2021 in Austria were included in this retrospective population-based cohort study. A strict inclusion criterion was the Austrian citizenship of the mother. The effect of month of birth (MOB) on birth length was estimated using a multivariable linear model adjusting for maternal educational level, newborn sex, gestational age, year of birth (YOB) of the newborn, and parity of the mother.

Newborn length varied by MOB, but there was also a temporal trend. In the birth cohorts up to 2004, the longest newborns were born in February, while from 2008 onward, the longest birth lengths were observed in the summer months.

In this Austrian population-based sample, birth length shows nonrandom fluctuations by birth month. These patterns, however, varied considerably over time.