Davi Peters had a feeling she was pregnant. Her breasts were tender, and felt different than when she had her period. The Hollis, N.H., resident took a pregnancy test, but it was negative. She figured her period was on its way and forgot about it. About five days later, she realized she hadn't gotten her period and took another pregnancy test. This time it was positive.

Peters describes her breasts as having an aching tenderness. "They also started to feel fuller right away, and my bras stopped fitting just after I found out I was pregnant," she says.

Simply put, breast tenderness happens due to increased hormone levels of estrogen and progesterone, says Dr. Shelley Binkley, an OB/GYN in Glenwood Springs, Colo., and author of DIY Baby! Your Essential Pregnancy Handbook (iUniverse, Inc., 2008).

The estrogen production causes a response of the mammary tissue within the breast that results in tenderness, says Michelle Collins, assistant professor of nursing, nurse-midwifery specialty, at Vanderbilt University School of Nursing in Nashville, Tenn. There is also a great deal more fluid and blood circulating within the pregnant woman's body. "This increase in circulation volume can result in engorgement of the breast tissue (fullness) as well as prominence of the blood vessels of the breast tissue, which is also why the pregnant breasts can look like they have a blue 'highway' map on them," Collins says.

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