Incidence of neonatal hip instability: are there seasonal variations?

We wanted to establish whether the season of birth affected the incidence of neonatally diagnosable hip instability in Eastern England. Data relating the numbers of maternities to the numbers of cases of congenital displacement of the hip diagnosed in neonates were analyzed month by month over a period of 8 years. The study covered 185,744 maternities and 154 cases of displaced hips during the period 1979-1986. While there were wide fluctuations in the monthly incidence, there was a striking excess of the malformation in late winter and early spring, i.e. in January, February, March and April, and a progressive decline in the succeeding months. The cause or causes of the seasonal variation are unknown.