Risk Factors Revisited

Now that we've discussed the Maternal-Fetal Conflict and seen the role of sFlt1 in the cascade of symptoms, let's take another look at risk factors. What leads to a placenta implanting poorly?

* a first pregnancy, or a first pregnancy with a new partner
While the risk of PE is around 5% for women as a whole, it's around 10% for first time mothers. Most women who get it once do not have it in subsequent pregnancies. Why?

One theory is that in a first pregnancy, a woman's body doesn't know how to react to the "foreign invasion" of the placenta, and so has a stronger immune response than needed. In subsequent pregnancies, Mom's body knows what to expect and doesn't attack as strongly. It's thought to be a similar process to desensitizing therapies done with allergies. This would fit with the increased risk in a pregnancy with a new partner. Mom was already "desensitized" to the previous father but reacts more strongly to the DNA of the new father.

Another theory is that the first placenta does a lot of work to remodel Mom's uterine arteries. Subsequent placentas can build on this work to make a deeper, better connection. One scientist describes it like a pair of pantyhose. Compare a brand new pair to one that has been worn: it's stretched out. It takes time to bounce back to normal. If Mom's immune response is just a little stronger than normal, if she had mild PE late in the pregnancy, it's a lot less likely she'll have it again, possibly due to this phenomenon.

* Age: teenagers and those over 40
Teenagers' bodies are still gearing up toward peak fertility. They aren't as prepared for pregnancy and are more likely to overreact in their immune response. On the other end, as we approach menopause, our bodies are gearing back down and are no longer as prepared to accept a placenta.

* Women who have chronic high blood pressure outside of pregnancy, including those with a predisposition for high blood pressure that has not yet shown up.
If you have high blood pressure outside of pregnancy, your risk of developing PE is 25%. If you have a predisposition for high blood pressure, the stress of pregnancy can bring it out, just like the stress of getting older or gaining weight. If your blood pressure goes up in the first trimester, it's generally assumed you have unmasked chronic hypertension rather than pregnancy-induced hypertension. The risk in this case is also 25%. It is thought, though, that the increased risk is less due to a poorly implanted placenta but rather to one that has been damaged by the higher blood pressure. This is called super-imposed preeclampsia. The cause is different, but the result is the same.

* Autoimmune disease such as lupus and rheumatoid arthritis
Having an autoimmune disease means your immune response is already more sensitive than it should be. It makes sense, then, that such women would also have a higher immune response to the placenta and would thus be more prone to PE.

* A blood clotting disorder such as Factor V Liden and MTHFR
Once again, it is thought that this risk factor might be due to super-imposed PE--the placenta may have implanted normally, but mini blood clots block off the artery connections, leading to poor nutrient transfer and the cascade of symptoms.

* A family history of PE, especially if either grandmother had it while pregnant with Mom or Dad
Research into the possibility of a genetic connection is in early stages and ongoing, but it seems very plausible that a mother with a strong limit-the-fetus gene or whose husband passes on a strong grow-a-big-fetus gene would be at greater risk.

* Carrying multiples
Do the math: more placentas=more chance of something going wrong. Also, more babies=greater demand for nutrients.

* Obesity
This may be less due to the obesity than to the tendency toward high blood pressure that often goes along with it, but improving BMI also seems to improve pregnancy outcomes as far as PE is concerned.

* personal history
After a mild case at term, the risk may be no higher than a first pregnancy (10%), but the earlier and more severely it struck, the greater the risk of recurrence. Having PE severe enough to require delivery before 36 weeks puts the risk of recurrence at 40%; before 28 weeks, 60%. Whatever issue caused PE the first time may still be in play in subsequent pregnancies. The good news is, in most cases, if PE does strike again it's likely to be milder and later.