Researchers at Hadassah Medical Organization in Israel have identified “natural killer cells” as a key component in answering why each pregnancy is easier and healthier than the first.

According to Ellen Hershkin, president of Hadassah, the Women’s Zionist Organization in America, Inc. The study results, published in the May 15th  issue of the medical journal immunity, are the product of six years of research led by Dr. Simcha Yagel, head of OB/GYN at HMO, and included analyzing tissue samples from more than 450 pregnancies.

Yagel’s research, a type of biology called epigenetics, studies the change in an organism caused by modifications of gene expression. Yagel and his team discovered a population of natural killer cells found in repeated pregnancies which has a unique transcription and epigenetic signature. They named them “Pregnancy Trained Decidual Natural Killer Cells.

“Our findings may provide an explanation as to why complications of pregnancy, especially the ‘great obstetrical syndromes’ (intrauterine growth and small birth size) are less frequent in repeated pregnancies,” Yagel said.

One possibility the researchers suggest is an artificial expansion or manipulation of natural killer cells during the women’s menstrual cycle prior to pregnancy.  That’s possible because natural killer cells with their unique “memory” function accumulate in menstrual blood.  According to Yagel, “The cells seem to be waiting for the next pregnancy.”

Natural killer cells are part of the body’s immune system and their name is derived from their ability to wipe out tumors and pathogen-infected cells.  These same cells are abundant in the human decidua, the lining of the uterus that forms the maternal part of the placenta during pregnancy.

Yagel’s goal is to develop a test to screen risk factors. He said, “by understanding how natural killer cells work, we can ask what’s missing in the first pregnancies and eventually develop a treatment.”

“Once again, HMO researchers are in the vanguard of providing life-saving, state-of-the-art medical research that is critical to the care, safety and wellbeing of women not only in Israel but around the world,” Hershkin said. “This new study furthers Hadassah’s mission to protect and empower women at one of the most critical and defining moments of their life.”