A government-funded study has shown that the survival chances of extremely premature U.S. infants have improved, although prospects for the ones born nearly four months earlier than the delivery date remain poor, Yahoo news reports.

"Our findings are cautiously optimistic. Progress is being made," said Dr. Barbara Stoll, the lead author and head of pediatrics at Emory University's medical school in Atlanta, according to Yahoo news.

The study was published in Tuesday's Journal of the American Medical Association.

The researchers studied hospital records from 1993 through 2012 for nearly 35,000 extremely premature babies born at 26 academic centers participating in a National Institutes of Health research network.

The infants included in the study were born at 22 weeks to 28 weeks of pregnancy, weighing about 14 ounces to just over 3 pounds.

The study has shown that in 1993, 6 percent of babies in the study born at 22 weeks survived long enough to leave the hospital in comparison to 9 percent in 2012. Of the 1,550 infants born at 22 weeks during the 20-year study, only 99 infants survived till the hospital discharge, and only 5 infants survived without major complications.

However, the study showed that the chances of survival, without major complications, climbed dramatically with just a few more weeks in the womb.  The survival chances of premature babies born at 27 weeks climbed from 29 percent in 1993 to 47 percent in 2012.

Improvements for the survival of the premature babies are likely to improve further due to changes in medical practices, effective infection control procedures, helping preemies breathe without the use of ventilators and the use of steroids for pregnant women before childbirth to boost lung growth.