My girlfriend hasn't had a period in 3 months. she took twofirst response tests and they were both negative. she took the tests 30 days after having sex and 36 days from her last period. could she be pregnant or not?

Doubtful: I would encourage her to have taken the two tests on two different days about a week apart. If she did this and both were negative, it is unlikely at that time frame that she is pregnant. . If she has been three months without a period, she is likely experiencing a anovulatory phase, and there are a number of reasons this could happen at her age. She should see her ob/gyn.

Not if negative : Home pregnancy tests are very accurate. They are the same tests used in a doctors office. They can detect a pregnancy 7-10 days after conception. If you doubt the result you can repeat it in 1-2 days. If you were pregnant the hormone level would rise enough for detection. After this much time clearly you are not pregnant. See your doctor for irregular periods.

Missed period: Missed period may not be caused by pregnancy only. If her tests are negative, she is not pregnant. She may not have ovulated which is known as anovulation. This will eventually lead to profuse uncontrolled vaginal bleeding. To prevent that, she will need to talk to doc to "jump start" her periods.

Agree w earlier : Responses. Reasons for missed/late periods: low body weight, obesity, marked weight ^ or v, over-exercise, endurance athletics, breast-feeding, ^ stress, illness, eating disorders (bulimia or anorexia), anovulatory cycle (no egg released), travel, hormone irregularities, drug use, meds (i.e., birth control) or medical problems. #1 reason is pregnancy.