A 10-year study examining 4.9 million births in the 1990s has found more evidence that there's a link between autism and the mother's age at conception. "The risk of having a child with full syndrome autism increases with maternal age," concluded researchers at the University of California, Davis, who examined data from all births in their state for the decade. The findings are published in the February issue of the journal Autism Research.

The link between the parents' age and children's health is not entirely new. Prior studies have indicated that babies born to older women have higher risks of birth defects, low birth weight and certain chromosome problems, such as Down syndrome.

A 2007 Kaiser Permanente study conducted in California reported that autism risk increased with both the mother's and father's age. An Israeli study based in statistics from 1980s had isolated only paternal age as being linked with increased risk for autism.

Dr. Max Wiznitzer, a pediatric neurologist at Rainbow Babies & Children's Hospital in Cleveland, Ohio, said the latest research had a far larger sample size. Autism is a growing disorder; the Centers for Disease Control and Prevention estimated that one in 110 children had the condition in 2006. But its causes remain unknown.

In the latest study, researchers found that mothers over the age of 40 had 51 percent higher odds of having children with autism compared with mothers between the ages 25 and 29. The father's age also played a factor, but only when he had a child with a woman under 30.

"When the mom has minimal age risk of an autistic child, we do see increased risks as dads get older," said lead author Janie Shelton, a graduate student researcher at UC-Davis.

It's unclear why the mother's age has more bearing in autism risk than the father's.

The study authors emphasize that while autism rates have risen 600 percent in the past two decades, older women having children contributed to only 5 percent more cases of autism.

As more women delay childbearing, it's important to keep the study in perspective, said Geraldine Dawson, chief science officer of Autism Speaks, the nation's largest autism science and advocacy organization.

"When we look at that dramatic increase [of autism] over the last two decades, there are multiple factors that have contributed to this," she said. "It appears that advanced parents' age, not just mothers but also father's, account for a very small portion of that increase."

Shelton said older mothers should not jump to conclusions. "I don't think a mom blaming herself is going to help us understand what's causing autism or help prevent further cases," she said. "I would urge parents not to blame themselves, regardless of what age they are."

Shelton and the co-authors obtained all birth records in California from 1990 to 1999 and then collected data from the state's Department of Developmental Services to count the number of autism diagnoses from children born during that decade.

How parental age increases autism risks remains unknown, but several hypotheses exist. Some suggest that the cumulative effects of the environment, changes to the autoimmune system, stress and reproductive technology may affect autism risk.

"As people age, we know there are changes to our DNA that occur," Dawson said. "There have been studies that show we have increased damage to our DNA as parents age. They're more likely to have children of low birth rate and more birth complications. It's not surprising that those factors would slightly increase the risk for autism as well as other neurological disorders."

Despite the lack of concrete answers, Shelton said, the findings offer some hints.

"It gives us some clues where to look biologically," Shelton said. "In an epidemiological study, age is a proxy for a lot of things. And so we're trying to further understand why age might be showing up as a risk factor, because we don't know the mechanism yet."

通过一项对490万二十世纪九十年代出生儿童的为期十年的研究，已经发现在孤独症和孕妇年龄两者之间存在关联。加州大学研究员Davis对加州过去十年间出生的儿童有关数据进行了仔细研究，断定说：“孕妇的年龄越大，其生育子女患孤独症的几率越大“。这个发现被刊登在二月出版的孤独症研究期刊上。

父母年龄与其孩子健康存在联系并非一个全新概念。以往的研究者已经指出过，大龄孕妇生育的孩子极易患出生缺陷、出生体重低、类似先天痴呆的某些染色体疾病。

kaiser permanente机构2007年在加州开始的一项研究宣称，父母任何一方的高龄都会增加其孕育子女患孤独症的风险。以色列以二十世纪八十年代数据为基础的一项研究，仅仅是把父亲年龄与罹患孤独症有关联的说法排除掉了．

俄亥俄州克里夫兰的彩虹婴幼儿医院儿童神经科医师Max Wiznitzer说，最近的研究已经发现大量的例子，孤独症是一种不断发展的机能失调；疾病预防和控制中心估计，在2006年，110个儿童中就有一个患孤独症的，但原因尚不清楚．

研究者最近发现，与年龄在25岁至29岁之间的孕妇相比，40岁的孕妇生育患有孤独症婴儿的机率要高出百分之五十一。父亲的年龄也是一个诱因，但只有当他与一个年龄在30岁以下的女人生儿育女时才会如此。

“我们的确注意到，当妈妈的年龄几乎没有生出罹患孤独症孩子的风险时，爸爸的年龄越大这种风险也越大”，一位Davis大学研究生Janie Shelton说。

尚不清楚为何在生出孤独症孩子这件事上，母亲的年龄因素要比父亲的年龄因素影响更大。

这项研究的发起人强调，在过去二十年间，孤独症的发病比率上升了百分之六百，其中，大龄孕妇生出孤独症孩子的比率只上升了百分之五。

随着更多女性推迟生育年龄，继续保持这项研究非常重要，美国最大的孤独症研究和救助组织——“孤独症之声”的首席执地官Geraldine Dawson说。

“我们看到，在过去二十年孤独症得以惊人增长，这是有着多种原因的”，她说，“表明父母年纪增长，不仅是母亲也包括父亲的年龄，在这种增长中所扮的份量很小。”

Shelton说大龄母亲们不应该过早地下结论。“我们认为一位母亲把问题归咎于自己并不能帮助我们发现导致孤独症的根本原因，也不能防止此类事情的再度发生。”她说，“我会劝说父母们不要自责，不管他们处于哪个年龄”。

Shelton和合著者取得了1990年到1999年期间加州所有婴儿的出生记录，同时从国家发展服务部门收集到数据，统计出这十年间有多少孩子自出生起就被诊断为孤独症。

父母年龄是如何加大孤独症风险的仍是未知数，但存在一些假设。有人认为，周围环境的长期影响、自身免疫系统的变化、压力和生育科学可能对孤独症几率有影响。

“随着年岁日长，我们知道自己的遗传基因会发生变化”，Dawson说，“已经有研究表明，年龄的增长对遗传基因有破坏作用。高龄人群可能会导致低出生率和更多的出生并发症。毫不奇怪，这些因素象其他的神经性失调一样，可能会稍稍增加患孤独症的危险。”

尽管没有明确结论，Shelton说，这些发现还是大有裨益。

“这些发现为我们在生物学方面的研究提供了线索”，Shelton说，“在流行病学研究中，年龄是许多事物的表象，因此我们正在深入探索为何年龄会越来越显现为风险因素，因为我们还不清楚其中的玄机。”