Scientists say that smoking may not just harm your fertility but also that of the next generation.
Men who smoke have a lower concentration of proteins in the testes that are essential for producing sperm, while women who smoke during pregnancy may be sowing the seeds of infertility in their unborn child, as two separate studies.
In one of the studies, researchers obtained 24 testes, from 37 to 68 day embryos after legally terminated pregnancies.
They found that the number of germ cells - responsible for forming eggs and sperm - was reduced by 55 per cent in fetuses from women who'd been smoking while pregnant. They also found a 37 per cent reduction in the ordinary or somatic cells in the embryos.
"We were very surprised that smoking so early in pregnancy has such profound effect on the number of germ cells in the gonads," New Scientist quoted co-author Claus Andersen from the University Hospital of Copenhagen in Denmark, as saying.
Results from a previous study on female fetuses showed a slightly lower corresponding reduction of germ and somatic cells - of 41 and 29 per cent respectively.
The researchers believe the reason the male embryos they studied showed higher sensitivity was because they were, on average, exposed to more cigarettes per day.
The authors do not known whether the reduction of germ cells is permanent - raising the question of whether women who smoke during pregnancy could harm the future fertility of their fetus.
Anderson says the next step is to expose cultured fetal testes to components of smoke thought to be harmful.
"We could expose the culture for a couple of weeks, and then continue [to develop] the culture without exposure and see if the cells are able to recover," he said.
In a second study, Mohamed Hammadeh from the University of the Saarland in Germany, and colleagues, examined the concentrations of two proteins called protamines P1 and P2, which are crucial to the formation of chromosomes during cell division. Changes in the concentrations of these protamines can have negative effects on male fertility.
The study, which compared sperm samples from 53 men who smoked more than 20 cigarettes per day with sperm from 63 non-smokers, found that smokers have 14 per cent lower concentration of P2, and a higher ratio of P1 to P2 overall.
They also found an increased level of "oxidative stress" in smokers - a chemical imbalance that is known to harm sperm DNA.
Since sperm can take months to develop before stored in the testes, the studies suggest that men hoping to reproduce may want to consider giving up smoking long before they try to conceive.