CHICAGO (Reuters) - Unintended pregnancies, which make up nearly half of all pregnancies in the United States, are increasingly concentrated among low-income women, a study showed on Wednesday.

Unplanned pregnancies have skyrocketed among poor women in recent years even as such pregnancies among their affluent peers have dropped, according to the study, to be published in the online edition of the journal Contraception.

Researchers from the Guttmacher Institute found the unintended pregnancy rate among women with incomes below the federal poverty line jumped by 50 percent between 1994 and 2006, the latest date available, from 88 per 1,000 to 132.

Meanwhile, the unplanned pregnancy rate among women with incomes at least 200 percent above the poverty line fell 29 percent from 34 per 1,000 to 24, the researchers found, using data from the federal National Survey of Family Growth.

Of the 6.7 million pregnancies tracked in 2006, some 49 percent were unintended, up from 47 percent in 1994, according to the women themselves.

The authors did not hypothesize why the disparity between poor and better-off women has grown but one of its authors, Lawrence Finer, the director of domestic research at the Guttmacher Institute, said the gap was consistent with broader income-based disparities in health care outcomes nationally.

The Guttmacher Institute works to prevent unintended pregnancy through support for the use of contraceptives and other forms of birth control, including abortion.

(Editing by Cynthia Johnston)