Oh, the impending guilt! There's a new scientific theory that babies actually do suck the life out of their poor mothers. According to evolutionary biologist David Haig, an expectant mother can suffer from a difficult pregnancy because she and her fetus are unconsciously fighting over a fixed amount of nutrients, as reported in today's New York Times.
"A fetus does not sit passively in its mother's womb and wait to be fed. Its placenta aggressively sprouts blood vessels that invade its mother's tissues to extract nutrients," writes Carl Zimmer. For example, Haig argues that preeclampsia, a complication occurring in about 6 percent of pregnancies in which the mother has late-term high blood pressure, is caused by a fetal attempt to produce a protein to direct more blood to the placenta.
Haig's work tries to answer the question of why some 529,000 women around the world die each year during childbirth and why 10 million others suffer pregnancy-related problems. Motivated by the work of Robert Trivers, an evolutionary biologist at Rutgers University, Haig says that the process stems from an age-old evolutionary conflict; mothers need to preserve resources to distribute among offspring, while babies have a better chance of surviving if they can claim a big percentage of them.
So in a battle that invariably will play out later in allowances and credit card bills, natural selection favors genes that "help children get more resources from their parents than the parents want to give," Zimmer writes. Haig figured the process must start in the womb.
But lest they be pushovers, mothers fight back by shutting down some of their genes in their children, Haig theorizes. That explains why in the past 15 years, scientists have found more than 70 pairs of genes in which one parent's copy doesn't produce a protein. Zimmer offers a lengthy explanation about how the process is evident in a gene called insulin growth factor 2 (Igf2). "Perhaps the mother's copy of Igf2 is silent because turning it off helps slow the growth of a fetus," he writes.
And if that doesn't work, she can may carry another gene that blocks the growth action of the father's. Researchers are trying to figure out how such imprinted genes create later medical disorders. And some of these genes are imprinted in the brain after birth, which can then influence how much milk a baby can wrestle from his or her mother.
Fine! I'll let my mother keep her dumb genes. I don't have time for them anyway. I'll be too busy looking for a Mother's Day card that reads: "Thanks for always being there when I was sad. Thanks for making me feel special. Thanks for letting me have your nutrients when you needed them most ..."