Actress Mandy Moore said she must have an "unmedicated birth" with her second child because she suffers from a rare blood disorder called immune thrombocytopenic purpura or ITP.

The star of the hit television series "This Is Us" was diagnosed with the condition characterized by an abnormally low blood platelet count after giving birth to her first child last year. The illness affects her blood's ability to clot and prevents her from having epidural to ease labor pains.

"My platelets are too low for an epidural," she said, according to Prevention, adding that giving birth the first time with son Gus was difficult. "But I can do it one more time. I can climb that mountain again."

According to the National Library of Medicine, ITP is a bleeding disorder in which the immune system, for reasons unknown, destroys platelets that are necessary for normal blood clotting. In children, the disease follows a viral infection. In adults, it is more often a long-term disease that can occur after a viral infection, with the use of certain drugs, during pregnancy, or as part of an immune disorder.

ITP affects women more often than men and is more common in children than adults. In children, the disease affects boys and girls equally. The National Organization for Rare Disorders says that ITP can occur from three months to over 100 years of age. The prevalence of adults suffering from ITP in the U.S. is 9.5 per 100,000 annually.

Research has found that ITP is a pretty rare condition in pregnant women, affecting one or two in 1,000 pregnancies. It's often associated with gestational thrombocytopenia, also characterized by a low platelet count, during the second half of pregnancy.

Moore shared on Instagram that she underwent testing for both conditions and was diagnosed with ITP when her platelet counts remained low after giving birth. According to Robyn Horsager-Boehrer, M.D., of the UT Southwestern Medical Center, low platelet counts during pregnancy can lead to the inability to have an epidural during labor due to a risk of epidural hematoma, in which abnormal blood collection in the spine can cause spinal cord damage.

Even if Moore has a vaginal or cesarean delivery, ITP increases the risk of bleeding and premature delivery.

Symptoms of ITP include heavy menstrual periods, easy bruising, and abnormal nose and gum bruising. Treatment options include steroids, high-dose globulin infusions, and other medications. While it's not clear if Moore underwent any of these treatment options, she shared with Instagram followers that she tried boost her platelet count by eating dark leafy greens and lentils, but still her platelet levels "weren't great," she said.

Moore, 38, said that during Gus' delivery last year, her platelets dropped dramatically, which kept her from getting an epidural. As her son's heart began to drop, Moore pushed harder.

"And it went from no baby to a full body out in seconds," she said.

For most patients, having mild-to-moderate low platelets won't change pregnancy care or a birth plan much, Horsager-Boehrer said.

"However, it's important to talk with your doctor about your risks for complications related to low platelets," she said. "If your lab results indicate low platelets, follow Moore's lead. Ask your provider to explain what that means to you."