For about a third of US mothers, the inter-pregnancy interval is less than one and half years, says a new study.

However, the typical time between pregnancies is about two-and-half years in the US and about a half of mothers space pregnancies between 18 months and five years, the findings showed.

The timing between a live birth and the next pregnancy, termed the inter-pregnancy interval (IPI), may increase the risk of pregnancy complications, such as preterm birth, low birth weight, and small gestational age (birth weight that is small for a given gestational age), the study pointed out.

Health experts recommend a minimum of 18 months between births. The researchers also found that about a while the older the mother is, the longer the spacing between a birth and her next pregnancy.

The study released by the US Centers for Disease Control and Prevention (CDC) is based on 2011 birth certificates from 36 states and the District of Columbia, representing about 83 percent of the nation's births that year.