Your private parts have a topography similar to the Blue Ridge mountain range.
Estrogen thickens the vaginal tissue and causes it to buckle slightly, creating many tiny folds called rugae (sounds sexy and vaguely European, doesn't it?). This provides more surface area for secretions and lubrication, and it also gives the vagina more stretch during intercourse and childbirth, explains Daniela Carusi, MD, the director of general gynecology at Brigham and Women's Hospital in Boston, a teaching affiliate of Harvard medical school. Estrogen decreases in women who just had a baby, are breastfeeding or are going through menopause, causing the ridges to flatten out and the vaginal tissue to become thinner and drier. This is one reason women in these life stages may experience discomfort during sex, says Carusi. (Fortunately, lubricants and topical estrogen can help.)