State Laws Limiting Abortion May Face Challenges on 20-Week Limit
Banning abortions after a specific point in pregnancy has been a popular trend in the states this year. Last week, GOP Gov. Rick Perry made Texas the 12th state to ban most abortions after 20 weeks.
But how states define the starting point for that 20 weeks may cause headaches for women and their doctors — and ultimately affect whether these laws pass constitutional muster.
Like all but one of the abortion bans passed so far in the states, the Texas law starts its 20-week calendar at fertilization. But that’s not the same as saying 20 weeks of pregnancy, because that’s not how doctors measure pregnancy.
“When we refer to the weeks of pregnancy, weeks of gestation, we measure pregnancy from the date of the last normal menstrual period,” says Dr. Daniel Grossman. He’s an assistant professor of obstetrics, gynecology and reproductive sciences at the University of California, San Francisco and a vice president of IBIS Reproductive Health, a reproductive rights advocacy group.
“For a woman who has a normal menstrual period, ovulation or fertilization would generally occur two weeks later, after that start of that normal menstrual period,” Grossman says. “The age of the embryo or the fetus is essentially two weeks less than the number of weeks measured from the last menstrual period.”
Last menstrual period, or LMP, is generally how doctors refer to the weeks of pregnancy. Forty weeks LMP is considered full term for a normal pregnancy, even though at that point fertilization occurred only 38 weeks before. So why do doctors use a measurement that’s so imprecise?
That standard developed in the old days before ultrasound was widely used, Grossman says. “The last menstrual period was something that was knowable and was measurable, whereas it wasn’t always known when fertilization took place.”
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