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【bio-news】警惕：男性生物钟30多岁开始奏效了
By LAUREN STREICHER, M.D.
Nov. 26 - A new report finds that men over 35 are twice as likely to be infertile as those under 25. And as a man's age increases, so too does the likelihood of potential genetic disorders for his unborn children.
It is well known that a woman's ability to conceive takes a dramatic dive as she approaches 40, but, what about the male biological clock?
Men are often the forgotten piece of the infertility puzzle, but recent research suggests that infertility or early pregnancy loss isn't always because of an aging egg.
A recent report from the Journal of the American Medical Association looks at past research to examine why aging men experience declining fertility.
It appears that men older than 35 are twice as likely to be infertile as men younger than 25.
As men age, both the number and quality of their sperm decline -- so older men become less likely to father a child and more likely to father a child with schizophrenia, Down syndrome, or other problems.
A recent study suggests that autism, an increasing problem with no known cause, may also be linked to paternal age because men 40 years or older are almost six times more likely to have a child with an autism disorder than men younger than 30.
Miscarriages also are more common as dad gets older.
It's not unusual for a woman to get her hormones, ovulatory function and fallopian tubes tested months before her husband has even had a basic semen analysis.
Given that 20 percent of couples are infertile because of abnormal or absent sperm and that 27 percent of infertile couples have a combination of male and female factors, it makes sense to evaluate a man's equipment, so to speak, sooner rather than later.
Sperm Quality, Not Quantity, Sometimes a Problem
While it's true that it only takes one sperm to impregnate an egg, sperm are not particularly skilled at the whole penetration thing.
While women only need to release one egg to successfully conceive, pregnancy is unlikely to occur unless there are millions of sperm swarming around it.
That's why the first step in an evaluation of male fertility is a semen analysis, to see how many of the little guys there are.
Counts greater than 20 million are considered to be normal.
Before a proud man with a count in the zillions alerts the media, he needs to keep in mind that even if the number is high, sperm quality is also a factor.
Every sample of semen has lots of sperm that are abnormal. If more than 85 percent of the sperm don't have heads, tails, or look funny in some way, it doesn't bode well fertility-wise.
In addition, if a sperm looks normal but is directionally challenged, the likelihood of finding its way down the fallopian-tube highway is limited.
Anything less than 25 percent to 40 percent forward motility reduces pregnancy rates. These are all factors doctors consider when running a semen analysis.
The Source of the Problem Sometimes Solvable, Sometimes Unexplained
There are four main causes of male infertility.
In roughly 10 percent to 20 percent of infertile men, an obstruction prevents sperm from traveling from the testis (where it is produced) to the urethra.
Roughly 30 percent to 40 percent of infertile men suffer low-sperm production as a result of testicular problems, resulting from infection, drugs, radiation or environmental toxins.
While hormone levels should be tested, they are rarely the problem.
Sometimes a low-sperm count is attributed to a varicocele -- dilated veins in the scrotum.
Varicocele repair was at one time a routine procedure thought to enhance male fertility, but is now highly controversial.
Studies show that the improvement in semen quality after varicocele repair doesn't always translate to increased pregnancy rates and can use up precious time, especially when a woman's biological clock is ticking.
The remainder of infertility is unexplained.
Men, unlike women, produce new sperm throughout their reproductive lives.
So while a 40-year-old woman is dealing with a 40-year-old egg, sperm is never older than 3 months old regardless of the age of the man.
However, that sperm becomes lower in quality as a man ages.
Aging men have declining levels of sex hormones, and it appears that these declining levels of testosterone have a significant impact on sperm production.
This well-publicized fact is certainly part of the reason that a number of men taking supplemental testosterone have increased 210 percent since 1999.
Supplemental testosterone is no magic pill, however. While higher testosterone levels potentially, but not definitively, result in improved sperm number and quality, supplemental testosterone may also be responsible for a number of health problems such as an increased risk of prostate hyperplasia, and possibly cancer.
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作者:admin@医学,生命科学 2010-09-26 18:32
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