Smoking while pregnant reduces the fertility of boys by affecting a key testis gene, researchers say.
It is already known smoking affects the future fertility of unborn boys, but it has not been understood how.
Aberdeen University researchers found significant reductions in the levels of a gene called DHH, which plays a key role in testicle development.
Small testicles are linked to low sperm counts, the Journal of Clinical Endocrinology and Metabolism reported.
The DHH (desert hedgehog) gene releases the DHH molecule in the testes, which tells other cells what to do and helps control normal testicle growth.
It is an often misunderstood subject. Male fertility is often determined by what happens in the womb
Dr Allan Pacey, fertility expert
The researchers examined 22 human foetuses between 11 and 19 weeks of pregnancy, looking at the levels of 30 genes key to the development of testicles.
There was no significant change in all but one case - the DHH gene. The study found the foetuses where women smoked 10 or more cigarettes a day had nearly half the DHH gene levels than non-smoking ones.
Problems
Lead researcher Paul Fowler said: "This is the first time that the gene DHH, which plays a key role in the male's normal development, has been linked to maternal smoking and fertility problems.
"Our research is still preliminary and a lot more work needs to be done."
Sheffield University expert Dr Allan Pacey, the secretary of the British Fertility Society, said: "It is an often misunderstood subject. Male fertility is often determined by what happens in the womb.
"Previous research has shown beef and pesticides in drinking water can have an impact on fertility.
"We already know smoking is not advised during pregnancy and this helps us to understand why it affects fertility."

Scientists say they may be on the brink of translating the thoughts of a man who can no longer speak into words after a pioneering experiment.
Electrodes have been implanted in the brain of Eric Ramsay, who has been "locked in" - conscious but paralysed - since a car crash eight years ago.
These have been recording pulses in the areas of the brain involved in speech.
Now, New Scientist magazine reports, they are to use the signals he generates to create speech software.
Although the data is still being analysed, researchers at Boston University believe they can correctly identify the sound Mr Ramsay's brain is imagining some 80% of the time.
CAUSES OF LOCKED-IN SYNDROME:
Brain injury
Drug overdoses
Stroke
Disease which damages nerve cells
In the next few weeks, a computer will start the task of translating his thoughts into sounds.
"We hope it will be a breakthrough," says Joe Wright of Neural Signals, which has helped develop the technology.
"Conversation is what we're hoping for, but we're pretty far from that."
Reading minds
Experts in the field of neuroscience agreed it was an exciting advance.
We are lot further away from a universal mind reading machine than some people hoped - or feared - we may be five years ago
John Dylan HaynesMax Planck Institute
"It hasn't come completely out of the blue," said Professor Geraint Rees, a neuroscientist at University College London.
"We have been moving towards decoding primitive vocabulary for a while now. But this is certainly an interesting development, although invasive techniques, where something is out in someone's brain, such as these will of course carry risks."
Reading people's minds remains a far-off prospect, however.
"There is a huge difference between a technique like this, which is able to pick up signals the subject wants to be picked up, and being able to delve deep into the mind," says Professor John Dylan Haynes of the Max Planck Institute for Human Cognitive and Brain Sciences.
"It's very exciting that we are starting to be able to translate some basic thoughts, but we are lot further away from a universal mind reading machine than some people hoped - or feared - we may be five years ago."