A new position statement by ACOG says that intake of 200 milligrams a day of caffeine will not increase the risk of miscarriage or preterm birth.

"For years, women have been getting mixed messages about whether or not they should have any caffeine during pregnancy," said a released statement by William H. Barth Jr., MD, chair of ACOG's committee on obstetric practice. "After a review of the scientific evidence to date, daily moderate caffeine consumption doesn't appear to have any major impact in causing miscarriage or preterm birth."
One cup of coffee therefore must be alright for women, but it must be pointed out that if they are drinking coffee outside of home then they must check the caffeine content.
Source-Medindia