Nearly all ectopic pregnancies—pregnancies in which the fertilized egg implants itself somewhere other than healthy tissue within the uterus—occur in a fallopian tube. But in women who’ve had a cesarean section, it’s possible for a future pregnancy to implant and develop in the scar tissue of the uterus where the C-section was done. This rare type of pregnancy is called a cesarean scar pregnancy, and it can be extremely dangerous. At times they are mistakenly referred to as “cesarean scar ectopic pregnancy.” However unlike the real ectopic pregnancies, this type of pregnancy may result in a newborn.

Estimated to occur in about one in every 2,000 pregnancies, the number of cesarean scar pregnancies has been steadily increasing over time, along with the number of cesarean deliveries themselves.

It’s not possible to predict who this will happen to. More than half of women who have a cesarean scar pregnancy had just one prior C-section. For reasons that aren’t clear, women who undergo IVF after a C- delivery are at a higher risk than women who conceive naturally.

Most cesarean scar pregnancies are discovered between five and 10 weeks into a pregnancy. You may not have any symptoms early on. The first sign may be vaginal bleeding, which could easily be mistaken for a miscarriage, so it’s important to make sure that your provider knows to screen you for a cesarean scar pregnancy. Rarely, women also have abdominal pain. Both pain and bleeding can worsen over time.

And time is not on your side. Early diagnosis is essential because a cesarean scar pregnancy can become life-threatening in a matter of weeks. Risks include the uterus rupturing and severe hemorrhaging, or dangerously excessive bleeding.

Some positive news: Transvaginal ultrasound has made diagnosing cesarean scar pregnancies easier and more accurate because it can detect where in the uterus the pregnancy is developing.

Once a diagnosis is made, many patients will be able to choose between two courses. The first is to continue the pregnancy with the understanding that complications could arise at any time, leading to a premature delivery and the possibility of an eventual hysterectomy, depending on how affected the uterus is.

The second option is ending the pregnancy. This may be done surgically in an operating room by aspiration or resection of the pregnancy, and requires general anesthesia and a short hospital sty. It also can be achieved by minimally invasive means such as a local injection into the embryo of methotrexate (the same drug used to treat rheumatoid arthritis and some forms of cancer) or other drugs.  A less invasive procedure was recently developed by experts in the department of obstetrics and gynecology at the New York University School of Medicine. It involves placing a special catheter (usually used in labor and delivery rooms to induce labor) with two balloons into the uterus and, under continuous ultrasound guidance, filling them with fluid to exert pressure against the embryo and stop the pregnancy. This method, tested in about over 50 cases has been shown to be more effective and safer than other methods. It is usually performed in an office setting, may only require sedation and does not necessarily need hospitalization.

Sometimes a combination of approaches is needed to ensure that the embryo is fully removed and, in turn, to prevent complications that could require a hysterectomy—that’s how serious a cesearean scar pregnancy is. What’s most important after being diagnosed with a cesarean scar pregnancy is to ask for an ob-gyn experienced in treating this type of pregnancy. Specialists working at a large medical center will likely see them more often than doctors at a small practice.

Being treated as quickly as possible gives you the greatest chance of preserving your uterus and your fertility. Most women will have a normal pregnancy after a cesarean scar pregnancy, but you’ll want to have an ultrasound as early as possible to make sure the new pregnancy is developing in the right spot.