Whaddaya mean, a mole?
Your ordinary pregnancy has reasonable odds of resulting in a baby. Moles, by contrast, almost inevitably lead to termination of the pregnancy (whether spontaneous or medical), and only in rare cases produce a living baby. In a complete mole, which is caused when a single sperm fertilizes an empty egg, no fetus is formed, but a placenta composed primarily of abnormal cells is created. The chorionic villi in the placenta grow unusually large (often described as "grape-like" in appearance) and the uterus is filled with clusters of these hydatidiform cells. Complete moles are frequently diagnosed by ultrasound, often after the mother experiences bleeding, when the grape-like cells appear instead of a fetus. If a complete molar pregnancy continues into the second trimester without detection, the mother may experience symptoms such as extreme nausea and/or a significantly enlarged uterus for dates.
A partial mole, on the other hand, occurs when an egg with a regular complement of chromosomes is fertilized by two sperm. (Slutty eggs, letting in two sperm! Don't they know their job is to let in one and only one!?!?!) The excess of chromosomes leads to the formation of a fetus, usually with fatal deformities, plus hydatidiform cells in the placenta. The most common symptom of partial molar pregnancy is spotting, but there are often no discernable symptoms. Partial moles are frequently diagnosed retroactively by pathological investigation of the fetus. Sometimes a partial molar pregnancy is suspected when beta hCG levels remain elevated after a miscarriage or D&C, but the diagnosis can only be confirmed by pathological review of tissues.
Unfortunately, finding out that your pregnancy didn't have a fetus or that the baby died from the chromosomal abnormalities is only the beginning of the fun with molar pregnancies.* The patient's beta hCG will be monitored frequently (up to two times a week) to verify that the levels are dropping at a consistent rate. This can take weeks or months. The goal is to reach a beta of zero. Once the beta drops to zero, monitoring will continue until three successive zero readings have been achieved. At that point, the molar pregnancy is considered resolved, although doctors recommend monthly hCG checks for 3 months to a year to detect a possible recurrence. The recurrence rate is incredibly small, but the possibility exists.
Oh, and the really fun part? You're not supposed to TTC again while the monitoring continues. That's right, you lose your baby and you lose as much as a year or more before you get to try again. Essentially, you can spend more time losing your baby than the average term pregnancy lasts.
That, in an extremely oversized nutshell, is your basic molar pregnancy. Stay tuned, because things can get wackier from here on out!
*In rare cases, a woman can have a twin pregnancy with one healthy baby and one mole. Such pregnancies can continue to term, but the risk of fetal death is significant. Even if one healthy baby is born, the mother still faces long-term monitoring and the possiblity of recurrence.