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Planning to have a child isn't easy at the best of times. Couples have to figure out when they're fertile each month, and perhaps seek treatment if they're not. Mothers have to worry about the effects of their own physical health and diet.

And now, a growing body of research suggests that even the month a baby is conceived in can have serious consequences, too. Infants conceived in June appear to suffer from birth defects at a higher rate than others, for instance. But babies born in the fall (conceived in November or December) tend to have more asthma. Even a baby's gender can be influenced by timing that's out of parents' hands: Recent research has shown the birth of baby boys dips nine months after stressful world events such as Sept. 11.

As researchers busy themselves filling in this veritable new zodiac, parents-to-be wonder how closely they should study the calendar.

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For now, most researchers say findings are more about their niche investigations than about setting practical guidelines. Still, the volume of information is hard to ignore. A sampling:

Research out of Indiana suggested a link between American babies being conceived between April and July and a higher risk of birth defects, including spina bifida, cleft lip and Down syndrome.

A study out of Bristol in Britain, released in February, found that late-summer and early-autumn babies in Avon were on average slightly taller (5 millimetres) and had thicker bones (12.75 centimetres squared) than those born in winter and spring.

A team in Nashville found a 30-per-cent increase in the risk of asthma for children born four months before flu season, in late fall and winter.

An Israeli study found that babies born in June and July had a 24 per cent greater chance of becoming severely myopic than those born in December and January.

Many studies have found that babies born in the Northern Hemisphere in February, March and April have a 5 to 10 per cent higher risk of schizophrenia.

While it may look as though scientists are trying to zero in on the perfect time to conceive, many are actually using the seasons to study the effects of everything from pesticides and flu viruses to sunlight and vitamin D. Any findings on the timing of conception are just a byproduct.

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Adrian Sayers, a medical statistician at Bristol University and lead author of the study that found taller and stronger babies are born in late summer, says he's looking into the way that vitamin D helps calcium make its way into the bones of children.

In previous research, he says, these effects were difficult to tease out from under various socio-economic factors. But the sun exposure of moms in their last trimester is common to all socio-economic positions, "no matter if you're rich or poor or eat lots of specific foods or not," he says. "It's a very elegant solution."

Other research hangs on a pregnant woman's exposure to potentially harmful substances in the environment, such as pesticides.

Paul Winchester, a professor of clinical pediatrics at Indiana University's school of medicine, recently found a spike in birth defects in summer babies, and suggested a link with levels of pesticides and other agrichemicals in surface water, since they peak at the same time. His previous work also found seasonal links to poor academic performance.

In the case of the asthma study, the birth month acts as a marker for elevated risk, one that can shift from year to year, depending on when flu bugs are strongest.

"There's no magic month [for avoiding asthma]" says lead author Pingsheng Wu of Nashville's Vanderbilt University. She says the critical finding of her work is that infants who are four months old when flu season strikes are more vulnerable than older babies to respiratory infections and, in turn, asthma.

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Still, the biostatistics researcher often finds herself deflecting questions about the perfect month to give birth.

"I always tell people August to September might not be good, but that's just for asthma," she says. "It may be perfect for avoiding other problems, though."

And some conditions are shared by whole generations. For example, researchers have found that stress hormones triggered by major world events that parents cannot plan for - a serious recession, say - can affect the gender of babies born nine months later.

Ray Catalano, an epidemiologist and professor of public heath at the University of California, Berkeley, discovered the phenomenon at work after Sept. 11, 2001, and during major cold-weather shocks in Sweden.

The female reproductive system appears to cull male fetuses in times of stress, he says, because boys are more likely to die between birth and reproductive age and they take more caloric and other investments to stay alive. So they may not be a safe bet, biologically speaking.

While Dr. Catalano and others continue to study this phenomenon, he says it shouldn't concern the general public: "Is this something the average couple would even think about? It shouldn't be."

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Dr. Catalano says he is skeptical about almost all research that suggests one conception month or season trumps others. He sees most of the purported outcomes as just as far out of parents' control as the ones he studies.

"Right now it's in vogue; it's a fashion to argue that babies born at different times [have different outcomes]" he says. "It's what I call neo-astrology. It's finding these differences that could be attributed to dozens of things - and most parsimoniously to chance."

The bottom line, according to most researchers, is that parents should not worry about engineering the month of a baby's birth.

"You should have a baby when you're ready to have it," Dr. Sayers says. "Don't let the seasons predispose you to when you get pregnant or not."

But that doesn't mean that once the deed is done, your timing is entirely moot. Mothers-to-be may want to ask their doctors about vitamin D supplementation if their last trimester is dull and grey, Dr. Sayers says.

Some argue that the month of conception may indeed become a routine topic of prenatal discussion one day. For instance, women who appear at risk of pesticide exposure because they conceived in June may one day be able to counter possible effects with antidotes or vitamins, Dr. Winchester says. "This is the future."

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