研究：怀孕时胎盘越重 患乳腺癌危险越高
  Placenta Size May Affect Breast Cancer Risk

摘要：
    瑞典一项最新研究结果显示，女性在怀孕期间，胎盘越重，其在更年期之前患上乳腺癌的危险就越大。
    The heavier the placenta during pregnancy, the greater a woman's risk of breast cancer before menopause, a new Swedish study suggests.

    瑞典一项最新研究结果显示，女性在怀孕期间，胎盘越重，其在更年期之前患上乳腺癌的危险就越大。

    据美国“健康日”网站11月15日报道，负责此项研究的瑞典斯德哥尔摩卡洛林斯卡研究所的斯文·克纳廷吉乌斯教授表示：“同那些首次生产时胎盘重量不到500克的女性相比，那些首次生产时胎盘重量在700克左右甚至超过700克的女性，其患乳腺癌的危险要高38％。”

    克纳廷吉乌斯教授说，尽管科学家尚不清楚引发乳腺癌的具体原因是什么，但可以明确的是，某些激素在其中扮演了至关重要的角色。他还表示，在怀孕期间，女性体内的某些激素的水平会发生很大变化。他说：“怀孕期间，某些激素主要由胎盘分泌而来。例如雌激素三醇（一种雌激素）的水平会随着胎盘增重而增加。我们由此猜测，母亲患乳腺癌的危险也许会随胎盘重量的增加而加大。”

    为验证这一猜测，研究人员重新查看了1982年至1989年间，瑞典出生记录上登记的30余万妇女的相关资料，并对这些妇女进行了长期的跟踪调查。调查显示，30余万妇女中有不到1％的女性（2216名）患上了乳腺癌，其中绝大部分人（95％）都是在50岁之前确诊的。

    查看和研究结果证实了研究人员的上述猜测。那些怀孕时胎盘重700克或以上的妇女和怀孕时胎盘重量不到500克的女性相比，前者患乳腺癌的危险比后者要高出38％。另外，第一次怀孕时胎盘重量在500克至699克之间、第二次怀孕时胎盘重量至少为700克的女性，和连续两次怀孕时胎盘重量都不到500克的妇女相比，前者患乳腺癌的危险要比后者高出82％。两次怀孕时胎盘重量都至少为700克的女性患乳腺癌的危险要多1倍。

    研究人员同时还证实了以前的发现，即如果女性较早地成为母亲，那么其患乳腺癌的危险将相对较低。那些首次怀孕发生在19岁或更小的时候的女性，其患这种疾病的危险要少10％；而那些直到35岁或更大年纪才有第一个孩子的女性将面临着是平常女性20倍的患病危险。

    报道说，此项研究结果将发表在11月16日出版的《美国医学会杂志》上。

    The heavier the placenta during pregnancy, the greater a woman's risk of breast cancer before menopause, a new Swedish study suggests.

    "Compared with mothers whose placenta at first delivery weighed less than 500 grams, women whose placenta weighed equal to or greater than 700 grams had a 38 percent increase in risk of breast cancer," said study author Dr. Sven Cnattingius, a professor at the Karolinska Institute in Stockholm.

    Results of the study are published in the Nov. 16 issue of the Journal of the American Medical Association.

    While scientists don't know the exact cause of breast cancer, hormones clearly play a role in risk, according to Cnattingius. And during pregnancy, he added, hormone levels surge.

    "Hormones during pregnancy are primarily produced by the placenta. And the levels of, for example, estriol -- the most potent estrogen -- are reported to increase with placental weight. We therefore hypothesized that a mother's risk of breast cancer may increase with placental weight," said Cnattingius.

    The placenta is the temporary organ that supplies nourishment to the fetus.

    To test this hypothesis, the researchers reviewed data from more than 300,000 Swedish women included in the Sweden Birth Register from 1982 through 1989. The study included follow-up information on the women through 2001, or until they had been diagnosed with breast cancer or died.

    Fewer than 1 percent of the women -- 2,216 -- developed breast cancer. Almost all of the women -- 95 percent -- were diagnosed before the age of 50.

    The study confirmed the researchers' hypothesis. Women whose placentas weighed 700 grams or more had a 38 percent increased risk of breast cancer, compared to women whose placentas weighed less than 500 grams, according to the study. A woman with a placenta weighing between 500 and 699 grams in one pregnancy and 700 grams or more in another pregnancy had an 82 percent increased risk of breast cancer, compared to women who had two pregnancies with placentas weighing less than 500 grams.

    When compared to women with two pregnancies with placentas weighing under 500 grams, women whose placentas weighed 700 grams or more in two pregnancies had double the risk of breast cancer, the researchers said.

    "The risk of (predominantly premenopausal) breast cancer among mothers increase with placental weight," Cnattingius concluded.

    The researchers also confirmed previous findings that an earlier maternal age was somewhat protective against breast cancer. Those whose first pregnancies occurred when they were 19 or younger had about a 10 percent reduced risk of breast cancer, while those who waited until they were 35 or older to have their first child faced 20 times the average breast cancer risk.

    Commenting on the placenta findings, Debbie Saslow, director of breast and gynecological cancer control for the ACS, said, "This is an interesting study, but it's not cause-and-effect, it's an association. The risk reported is a really small relative risk, and it's not going to change any lifestyle recommendations."

    The only women who might want to consider these findings are those who have tested positive for breast cancer genes, Saslow added.

    If you know you're already at a high risk for breast cancer, and if you don't feel strongly about when in your life you have children, it might be better to have them earlier, she said. But, that doesn't mean you couldn't have children later in life either, she added.

    What women can do to help prevent breast cancer is get enough exercise, eat right and don't drink alcohol excessively. Besides potentially lowering your breast cancer risk, you'll also lower your risk of heart disease and diabetes by taking these steps, she said.

    More than 200,000 American women are diagnosed with breast cancer each year, according to the ACS. Over a lifetime, 13 percent of U.S. women will develop the disease, according to the society.