Mothers of small stature have twice the risk of having a preterm or premature birth than their tall counterparts, says a new study.

To have the biggest chances of survival, babies need to be delivered only when they've reached full term, which is defined as 39 to 40 weeks of gestation. Otherwise, preterm birth, including extreme preterm (no more than 32 gestation weeks), may result in a variety of complications including the baby's vulnerability to childhood infections.

However, researchers from the Uppsala University and the Liggins Institute of the University of Auckland found out that there's a strong direct correlation between the likelihood of delivering babies before they reach full term and maternal height.

While tall mothers or those who are at least 179 centimeters (70 inches) in height have an increased risk of preterm and extreme preterm birth of 4.7 and 0.5 percent respectively, smaller women with a height of 155 centimeters (61 inches) or less have their odds at 9.4 and 1.1 percent.

Further, "decreasing height was associated with a progressive increase in the odds of having an infant born preterm," said the study, that is, the length of days is shortened by 0.2 days for every centimeter decrease of height.

The link remains strong even if other factors such as body mass index (BMI) and the women's age have been considered.

These results, now found in PLOS ONE on April 21, are based on the 1991 to 2009 data obtained from over 190,000 women in Sweden with Nordic origin and with ages at least 18 years old.

It's unclear how maternal height influences preterm birth, but there are a couple of theories. A 2015 study shows that it might have something to do with genetics and "anatomical constraints."

"Short mothers tend to have less space for the babies to grow before birth, and this seems to lead to premature delivery in some women," said Dr. Jose Derraik, lead author.

Meanwhile, preterm birth may be related to women's maternal age, level of affluence, and marital status.

Although there are limitations to the study, the key takeaway of the study is that perhaps it's time to include the maternal height as one of the possible risk factors for preterm birth.