New Yorkers have another thing to brag about: an expert says they are to than their sperm is swimming along strong as male fertility rates plummet elsewhere in the US.

Researchers studied 11 years of data on semen quality from sperm donors in six cities, Los Angeles and Palo Alto in California, Houston in Texas, Boston in Massachusetts, Indianapolis in Indiana, and New York's capital.

Consistent with a worrying global trend, they saw steady drops in total motile sperm count across the board - except for in the Big Apple.

Peter Schlegel, a New Yorker and president-elect of the American Society of Reproductive Medicine (ASRM), said: 'The exceptionalism of New York sperm donors is intriguing, but maybe not so surprising. New Yorkers tend to be physically active and our water system provides some of the cleanest and highest quality water in the US.

'We also have the best pizza.'

New Yorkers, notoriously proud of their pizza and water (used to make bagels), can now brag some more: they may be the reason New York sperm quality is not dropping like the rest of the world, a doctor joked

It's true that contaminant-free water and good exercise boosts a man's chance of good fertility (though no studies have been done on the relationship between the quality of both sperm and pizza).

However, Dr Schlegel admits the real reason for New York's sperm quality, and what is driving the 'concerning' trend of sperm count declines elsewhere, is a bit of a mystery.

The new data compiled and analyzed by researchers at the Icahn School of Medicine at Mount Sinai, California Cryobank, and the Reproductive Medical Associates of New York is being presented this week at the ASRM conference in Denver, Colorado.

They looked at total sperm count, concentration, and total motile sperm count () in 124,107 samples from 2007 to 2017.

In New York, there were no significant changes in quality. In Boston, there were declines in all categories except total motile sperm count - a heartening finding for the notoriously grump city, since TMSC is perhaps the most important factor.

Elsewhere, there were declines on all three fronts, even in famously healthy, active, happy and rich LA and Palo Alto.

The findings coincide with another paper showing sperm counts in both the US and Europe have plummeted in the last 15 years.

The proportion of men who can only conceive using IVF is on the rise, while the proportion of men with a 'normal' sperm count (over 15 million sperm per milliliter of semen) drops.

Some researchers have bandied about the phrase 'spermageddon', and Israeli scientists last year warned that the steady 40-year decline in sperm quality could pose a risk to the existence of the human race.

Dr Schlegel says our best bet is looking at lifestyle and environment (though both are hardly things traffic- and takeout-filled New York City does best).

'The trend toward lower sperm counts in this study is concerning,' Dr Schlegel said.

'Whether the causes underlying it are environmental or lifestyle-related, they will be difficult to parse out.

'Pollution, endocrine disrupting chemicals, poor exercise habits and convenient, yet nutritionally poor, dietary choices could all play a part.'

He added: 'Men planning to conceive should do what they can to achieve their best overall health to optimize their sperm quality.'

Or just head to New York City for a dollar slice.