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Women who give birth may be biologically "older" than women who don't, a new study suggests.
一项新研究发现，生过孩子的女性在生理上会比那些没有生过的更老。
For the study, the researchers analyzed information from 1,556 US women ages 20 to 44 who took part in a national survey from 1999 to 2002, which involved giving blood samples.
在该研究中，研究人员们分析了20-44岁之间的1556名美国女性信息，她们参加了1999年至2002年期间的全国调查，并给出了血液样本。
The researchers looked at the genetic material inside the women's cells, specifically the length of their telomeres.
研究人员们观察了这些女性细胞内的遗传物质，尤其是她们端粒的长度。
These are caps on the ends of chromosomes that protect the chromosomes from damage.
这些位于染色体末端的小帽子能够保护染色体不受损。
Telomeres naturally shorten as people age, but the structures don't shorten at the same rate in every person.
人们在变老的过程中，端粒也会自然缩短，但这个结构在每个人身上变短的速度均不同。
The longer a person's telomeres are, the more times their cells could hypothetically still divide, research has shown.
一个人的端粒越长，他们的细胞理论上能够分裂的次数就越多。
Thus, telomeres are considered a marker of biological age — that is, the age of a person's cells, rather than the individual's chronological age.
并且，端粒也被科学家们视为生理年龄的标记，即一个人的细胞年龄，而不是他们的实际年龄。
Women in the survey who said they'd given birth to at least one child had telomeres that were about 4 percent shorter, on average, than those of women who'd never given birth.
在研究中，至少生过一个孩子的女性端粒长度平均比那些未生产过的女性短百分之四。
The findings held even after the researchers took into account other factors that could affect telomere length, including the women's chronological age, body mass index and smoking habits.
即便研究人员们考虑了其它能影响端粒长度的因素（比如她们的实际年龄、身体质量指数和吸烟习惯），这些发现依旧站得住脚。
These findings suggest that a "history of live birth may be associated with shorter telomeres,"
这些发现表明生产史可能与更短的端粒之间存在着联系。
The researchers wrote in their abstract, which was presented at the meeting of the American Public Health Association in Denver.
该研究的概述在美国公共卫生协会于丹佛市举办的会议上发布。
The study was not designed to determine the reason behind the link, the researchers said.
该研究并未探究这一联系背后的原因，
But one hypothesis is that having children increases stress levels, and high stress has been linked with shorter telomeres, the scientists said.
但有猜测认为生孩子会增加女性的压力，而高压与更短的端粒有关。
"It is possible that pregnancy, birth and child-rearing can induce chronic stress, leading to shorter telomere length perhaps through an inflammatory pathway," said study researcher Anna Pollack, an assistant professor and environmental and reproductive epidemiologist at George Mason University.
美国乔治梅森大学的助教、生态环境与生殖流行病学家安娜.波莱克参与了该研究，她表示：有可能怀孕、生产和抚养孩子带来了慢性压力，并通过炎症通路缩短了端粒长度。
However, because the survey was conducted at a single point in time, the researchers cannot determine which came first in the women's lives — giving birth or having shorter telomeres, Pollack said.
然而，由于调查是在某个时间点开展的，研究人员们无法确定生孩子与更短的端粒长度之间，哪一个先出现。
It's also possible that for some yet-unknown reason, women with shorter telomeres are more likely than women with longer ones to have children, Pollack said.
波莱克说，有可能出于某些未知的原因，端粒更短的女性比端粒更长的女性更有可能生孩子。
More studies are needed that follow women over time and measure the length of their telomeres before, during and after pregnancy, she said.
她说，还需要进一步的追踪研究来确定女性的端粒在生孩子之前、怀孕期间和产后的长度。
"It would be interesting to see how telomere length changes during pregnancy, after birth and during the child-rearing years, and how these changes compare to women who do not have children," Pollack said.
波莱克说：观察端粒长度在孕期、产后和养育孩子过程中的变化，并将其与没有生过孩子的女性比较是一件非常有趣的事情。
