A wide-ranging analysis of over 270,000 pregnancies between 2004 and 2009 has found that the time of year that children are conceived can have a significant effect on successful birth rates.

Most notably, the researchers say the best time of year to start planning a family is December, with babies conceived in that month enjoying a markedly better chance of being born healthy. According to the study, December conceptions result in three extra surviving babies per every 200 pregnancies.

"There are a lot of things we are finding that are seasonal and very disturbing," said one of the researchers, Paul Winchester of Indiana University, as reported by Sarah Knapton at The Telegraph. "We have seen significant seasonal differences in reproduction. Valentine's Day is one of the least likely times to conceive a baby, whereas Christmas seems a very positive time."

For those who want to welcome a child into the world, Winchester says the middle of the year is not the best time to start, based on an examination of the statistics.

"June is a toxic month," he said. "The June effect was something that we saw develop at a very early stage. White mothers have the lowest survival rates in June and significantly shorter pregnancies, with premature babies."

While the researchers don't know for sure what factors cause the June risks and the December benefits, seasonal variations in the modern world are likely to be tied to agricultural elements, according to Winchester.

"It has been my suspicion that this is not accidental but may have some biological basis. Vitamin D levels and pesticides might be relevant factors," he said. "If you want to avoid a birth defect or a premature birth then it might be worth avoiding June. Other studies have shown that spina bifida incidence and sudden infant death peaks in June."

The findings, presented at the annual meeting of the American Society for Reproductive Medicine, follow the publication of recent research by the University of Cambridge in the UK, which found that babies born during summer are more likely to be healthy adults.

A groundswell of current research looking into the relative health of babies born at different times during the year might make some prospective parents a bit worried, but it's also worth keeping your fears in check.

"It's important not to get overly nervous about these results because even though we found significant associations the overall disease risk is not that great," said Nicholas Tatonetti, a researcher at Columbia University in the US, in relation to his team's study on the health implications of birth months. "The risk related to birth month is relatively minor when compared to more influential variables like diet and exercise."

The study by researchers at Indiana University may warrant deeper consideration, however, as it's looking at successful birth rates (not long-term health affected by many other variables). The research is not as yet published, but many will be interested in reading the findings once they become available.