Men considering having vasectomies have been warned it may not be possible to successfully reverse them.
The researchers looked at sperm samples from the men
Researchers from Queen's University, Belfast, found men who had been through the procedure had a much lower sperm count compared to men who had not.
They told the British Fertility Society conference fertility treatment was also less successful for the men.
They said the research was significant as many men wanted reversals so they could have second families.
Scientists studied 21 men who had vasectomies for contraception after having children more than 10 years ago.
They found that their sperm count was about three times lower than that of the 39 non-vasectomised fertile men assessed, at 3.6 million sperm per gram of biopsy tissue compared to 11.2 million sperm per gram.
The team also found that couples in which the man had undergone ICSI fertility treatment - in which a single sperm is injected into the egg to fertilise it - were less than half as likely to get pregnant as couples with other infertility problems.
But they say that, as only 17 post-vasectomy couples and 37 other infertile couples were assessed in this part of the research, further research needs to be carried out.
Dr Carmel McVicar, who worked on the research, said: "We did not expect to see this reduction in sperm count or pregnancy due to previous vasectomy and ongoing studies are attempting to decipher the reasons for it.
"In the past, a vasectomy was for life. Now men attend our clinic every week wanting to have a second family with a new partner.
"Men who are considering vasectomy certainly need to think very carefully about the long-term consequences to their future fertility."