01/30/2012 05:21 pm ET Updated Mar 31, 2012

Pregnant Mysteries

Anyone who has had a miscarriage -- or even worse a several pregnancy losses -- can't help but ask herself, "what is wrong with me? Why can't my body hold onto a baby?"

Oddly enough, scientists are asking just the opposite. They are wondering why pregnancies stick at all. I mean, if you think about it, your body should react to a fetus precisely the same way it reacts to a germ or even a tumor. It should rev up its immune system and start attacking.

The way the immune system works is that we are able to distinguish ourselves (our internal organs) from strangers (bacteria and viruses). A fetus is half ourselves and half someone else. The huge question is why we have allowed these half-foreigners to survive at all?

Recently, an eclectic group of scientists -- including those who study embryology, the placenta, genetics as well as the immune system -- are joining forces to figure out what humans have been doing ever since Eve: getting and staying pregnant. The answers will not only shed a glimmer of light on the fundamentals of gestation but should offer clues to help women who suffer from miscarriages and other women who suffer from pre-eclampsia, dangerously high blood pressure.

"This is a battle, an absolute battle. It's a war zone," said Yale University's Harvey Kliman, MD, PhD, who investigates the placenta. That's the way he talks about the pregnant body and the precarious relationship between mother and fetus. His recent study, published in the Journal of Reproductive Sciences this fall, suggests that a specific protein in pregnant women acts as a decoy of sorts to trick the mom's immune system to stay away from the baby.

Kliman claims a protein, called PP13 -- short for placenta protein 13 -- sends the mother's destructive immune cells far away from the growing fetus. In essence, as he puts it, it's like robbers who wreak havoc in the neighboring grocery store so they can steal the loot from the bank. It's a cute metaphor, but may not convince experts that the placenta proteins truly work this way.

Yet, Kliman points to areas of dead tissue that he spotted in the placenta of women who had abortions. He found it utterly "mindblowing" because typically you do not find huge areas of dead tissue in healthy organs. He then discovered PP13, the placenta protein, flooding the dead tissue. He suspects a connection. He suspects the PP13 drives immune cells to one area of the placenta -- away from the growing fetus -- so the fetus can continue to grow.

The PP13 theory is one of many circulating among experts.

Dr. Koji Yoshinaga, the program director of the reproductive sciences branch of the of the National Institute of Child Health and Human Development, said there are huge gaps in our knowledge about what really happens to during pregnancy to allow babies -- these foreigners -- to survive within us. We know that hormones of pregnancy change our immune cells. Perhaps the surge of progesterone or prolactin -- two hormones that increase during pregnancy -- calm immune responses near the fetus.

It's a huge balancing act, said Yoshinaga, between the mother's body revving up certain systems to nourish baby and staunching other systems that could hurt the baby. Among other things, his team is focusing on so-called BIEFs, or blastocyst implantation essential factors. He believes these factors signal to the mother that her body must provide nutrients without attacking. The earliest days of pregnancy -- when the first cells of the embryo are snuggling into the placenta -- are the most important to begin this tricky balancing act.

And while no one yet has the solution, the good news is that for the first time, experts in seemingly diverse fields (immunology and endocrinology) are spearheading joint projects to solve this age-old mystery.