Many women who feel nausea and fatigue mistakenly conclude that they're pregnant. Then Aunt Flo arrives, and the nausea and fatigue disappear. While frustrating, phantom pregnancy symptoms aren't necessarily uncommon – just ask anyone suffering through the two-week wait to do a pregnancy test.

But for some, the symptoms are so realistic and long-lasting that the women remain convinced they're pregnant for nine months or even longer – despite the fact that there is no baby.

Pseudocyesis, or false pregnancy, is extremely rare. "I've only seen two cases in my 30 years of practice," says Norman Duerbeck, a high-risk obstetrician at Sharp Mary Birch Hospital in San Diego.

Women with pseudocyesis may stop menstruating and have distended abdomens – probably caused by gas, says Duerbeck. Their hormone levels may rise, and their breasts can become engorged, sometimes even releasing colostrum. Some women develop health complications associated with pregnancy, such as preeclampsia. False pregnancy can even result in contractions.

What causes this strange condition? In some cases, an entirely unrelated physical or mental health condition is to blame. Ovarian tumors and severe depression, for example, can lead to both elevated hormone levels and the cessation of periods.

In most cases, however, the cause is psychosomatic – that is, a woman wants so badly to be pregnant that her brain creates changes in her body. Sadly, the condition is frequently triggered by trauma, such as multiple miscarriages or the death of a spouse. "It's very difficult for patients to accept that the pregnancy is not real," says Duerbeck. "They might even 'see' a fetus on the ultrasound screen."

Thankfully, there are fewer cases of pseudocyesis today than before, perhaps due to the availability of early pregnancy tests or perhaps, as Duerbeck suggests, because the role of women in society is less tied to motherhood than it was in the past. If there's one thing the disorder can teach us, it's that mind and body are strongly connected.