Luteinizing unruptured follicle syndrome occurs when the follicle develops but changes into the corpus luteum without releasing the egg. It may be more likely to occur in women with polycystic ovaries since the ovaries become tougher and it is harder for the egg to escape the follicle and the ovary. In some cases of LUFS, a thermal shift will be visible on a BBT chart, and a progesterone draw seven days after suspected ovulation may show a borderline response (between 5 and 15 ng/ml). The only absolute test of ovulation is pregnancy. The use of non-steroidal anti-inflammatory drugs such as ibuprofen and naproxen sodium (Advil and Aleve), as well as adult-strength aspirin, may contribute to LUFS. Therapeutic low-dose aspirin, also called baby aspirin, should not effect ovulation.