Monday, May 14, 2012

The Maternal-Fetal Conflict

Before I begin this post, I want to reiterate my disclaimer: I am neither a doctor nor a research scientist. I am a woman who has experienced the devastating effects of preeclampsia three times. I write about preeclampsia as I understand it. Also keep in mind that research is ongoing. What we understand today may be adjusted, altered, or even thrown out entirely as new information is discovered.

One of the most exciting areas of research into pregnancy complications in recent years has to do with the Maternal-Fetal Conflict. First proposed by David Haig, it has to do with the idea that mother and child have different survival needs.

Picture taken from Wikipedia
Let's start at the beginning. Sperm meets egg, the fertilized egg starts multiplying and dividing, and the zygote moves down the fallopian tube to the uterus arriving about 5 days post conception (which, due to the bizarre way we count pregnancy, is around the end of the third week). At this stage, it's called a blastocyst. The blastocyst contains a clump of cells called the embryoblast, which will become the actual baby, surrounded by the trophoblast, which will become the placenta and amniotic sac. Interestingly, all of the maternal DNA in the trophoblast has been switched off and all of the paternal DNA has been switched on. This means the placenta is essentially all father.

From a purely biological perspective, the goal of reproduction is to pass on your genetic material. For the father, and therefore the placenta, the bigger the baby, the better the chances of survival. As far as the placenta is concerned, a 100 pound baby sounds awesome. Thus, the placenta digs deep into the uterus, shunting crucial nutrients and oxygen from the mother's blood supply to the baby.

The mother, on the other hand, wants a baby big enough to survive BUT small enough to fit through the birth canal so that she, too, can survive to reproduce again. Our bodies are amazing and can expand to birth an 8, 10, even 12 pound baby. But let's face it--a 100 pound baby simply isn't going to fit. Thus, the mother must limit the placenta in some way to ensure her own survival. The mother's immune system attacks the placenta as a foreign invader, preventing it from attaching too well and from comandeering too many nutrients. 

It's a very delicate balance to grow a baby that is big enough but not too big. Dr. Haig suggests a normal pregnancy is like a tug of war: both sides are pulling on the rope, but the ribbon in the middle doesn't move if each side pulls equally. However, if one side starts to win the tug of war, if the survival of the other side is threatened, it will employ strategies to keep itself safe. He theorized that the fetus must be sending some signal to the mother that triggers the cascade of symptoms known as preeclampsia. This theory was confirmed by another doctor, kidney specialist Ananth Karumanchi, who discovered elevated levels of a protein called sFlt1 in women with preeclampsia.

Tomorrow I plan to explore the role of sFlt1 in preeclampsia.

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