For nearly 40 years, women have been delaying childbirth longer and longer, partly to launch careers. Now, this trend may be ending.

For the first time since government records have been kept, the average age at which women have their first babies posted a decline -- according to newly released data from the National Center for Health Statistics. Mothers' mean age at their first childbirth fell to 25.0 years in 2006, the most recent figures available, from 25.2 in 2005. Women ages 20 to 24 led the shift, with a 5% increase in the rate of first births.

A one-year reversal doesn't make a trend, of course. But the study lends weight to anecdotal evidence that young women are tuning in more closely to their biological clocks. 'It's the first time it's ever gone down, and certainly that's noteworthy,' says Brady Hamilton, co-author of the study.

Other factors are at work too, including rising numbers of Hispanics, who tend to start families sooner, says Steven Martin, an assistant professor of sociology at the University of Maryland. A 4% rise in the rate of first births to older teens, ages 15 to 19, is also playing a role. And the sheer size of the baby boomlet generation, now entering the child-bearing years, may be skewing new mothers' mean age lower.

But some experts also see a shift in attitudes. More young women today just assume they'll have both a career and a family, and on their own timetable, says Stephanie Coontz, director of research for the Council on Contemporary Families. Young women feel less compelled to spend a decade proving themselves on the job before kids, she says.

Sarah Distel, an at-home mother in Oxford, Ohio, and her husband, Christopher, a research technician, had their two children, now 1 and 3, in their mid-20s. Ms. Distel, a college graduate, sees her generation as unique. 'We weren't fighting for careers like the older generation. It was something we take for granted,' she says. After watching the struggles she has seen older moms face juggling established careers and child-care, Ms. Distel has decided to wait to launch her own planned career in library administration until her children are in school.

Candace Parker, the biggest star in women's basketball, shocked league officials and fans last month by announcing she was pregnant -- at the age of 22. If anyone had a career reason to defer child-bearing, Ms. Parker did; she was the 2008 league MVP and Rookie of the Year for the Los Angeles Sparks and had become the face of the sport in various marketing campaigns.

But after years spent striving to please others, says Ms. Parker, who is married to Sacramento Kings forward Shelden Williams, 'this decision obviously was for myself and my husband and my family.' Ms. Parker's parents were in their 30s when she was born, and 'I kind of missed out' on shooting hoops with her dad and other shared play, she says. She wants her children to have the benefit of young parents.

Close bonds with their own grandparents are important to young women, too. Heather Allen of St. Cloud, Minn., an art teacher whose husband is still in college, had her first baby at 21. She wants her children, now 4 and 1, to know their great-grandmother, 76; she considers extended family 'part of the village' it takes to raise a child.

It's unclear whether the country's current economic woes will affect the move toward younger child-bearing. Historically, recessions have reduced family size, but their impact on the age at which women start families is less clear. In any event, Dr. Martin says, 'demographers will continue paying very close attention' to whether a cultural sea change is under way.

在将近40年里，女性一再将生儿育女的年龄推迟，部分人是出于事业上的考虑。如今，这一趋势可能就要改变。

据美国国家健康统计中心(National Center for Health Statistics)发布的最新数据，自政府保存相关记录以来，女性的平均初产年龄首次出现回落。初产妇的平均年龄从2005年的25.2岁下降到2006年的25岁（这也是可以获得的最近数据）。年龄在20岁-24岁之间的女性是转变的主体，初产妇增加了5%。

当然，一年的转变尚不足以形成趋势。不过这项研究增加了此类个案证据的份量：年轻女性越来越顺应自身的生物钟。研究报告合着者汉密尔顿(Brady Hamilton)说，这是初产年龄首次出现下降，当然值得一提。

马里兰大学社会学助理教授马丁(Steven Martin)说，其他因素也在发挥著作用，包括拉美裔人的增多，这些人往往更早结婚生子。年龄在15岁-19岁的年轻女性中初产率上升了4%，这也起到了一定的作用。80后和90后女性现在都进入了生育年龄，这可能将女性的平均初产年龄拉低。

但一些专家认为，心态的转变也发挥了一定的作用。当代家庭协会(Council on Contemporary Families)研究负责人库恩兹(Stephanie Coontz)说，今天，有更多的年轻女性认为她们可以按照自己的时间表成家立业。她说，年轻女性感觉没有那么迫切要在生孩子之前花上10年时间证明自己的工作能力了。

俄亥俄州牛津市的全职妈妈萨拉•迪斯特尔(Sarah Distel)和她的研究技术员丈夫克里斯托弗(Christopher)二十四、五岁的时候生了两个孩子，如今两个孩子一个1岁、一个3岁。萨拉是大学毕业生，她认为自己这一代人独一无二。她说，我们不像老一辈人那样为事业拼死拼活。我们认为这是理所当然的。在看到比她年长的那些妈妈们在小有成就的事业和孩子之间忙得手足无措，她决定等到孩子上学后，再开始自己计划的图书馆管理工作。

女篮巨星帕克(Candace Parker)上个月宣布自己怀孕了，这让联盟的官员和她的球迷都震惊了，她只有22岁。帕克绝对有理由为了事业而推迟生孩子。她是2008年美国女子职业篮球联盟最有价值球员(MVP)，也是洛杉矶火花队(Los Angeles Sparks)的状元秀，在各种各样的营销活动中都成了女子篮球运动的代表人物。

不过帕克说，在多年努力让别人开心之后，这个决定显然是为了我自己、我的丈夫和我的家人。帕克的丈夫是萨克拉门托国王队前锋威廉姆斯(Shelden Williams)。她说，自己的父母三十多岁的时候生下她，她好像错过了和爸爸投篮以及其他可以一起分享的游戏。她希望自己的孩子能享受到年轻爸爸妈妈的好处。

与自己祖父母的亲密关系对年轻女性也很重要。明尼苏达州圣克劳德的希瑟•艾伦(Heather Allen)是位艺术老师，她的丈夫仍在上大学。她在21岁时生了第一个孩子。她希望自己的孩子们（一个4岁，一个1岁）能了解他们76岁的曾祖母。她认为一个大家庭是养育子女必需的元素之一。

尚不清楚美国目前的经济危机是否会影响女性降低生育年龄。历史上看，衰退会缩小家庭规模，不过对女性初产年龄的影响却没有这么清楚。无论如何，马丁说，人口统计学家都会继续密切关注生育文化潮流是否正在发生转变。