The surprising reason you should have sex twice a night if you're trying to conceive

Couples who are trying to conceive might benefit from having sex twice a night - and not for the obvious reason.

It's Fertility Week in New Zealand, where we recognise that getting pregnant isn't as easy as stopping your pill and hoping for the best.

A new study out of Shengjing Hospital in China offers some advice for couples who are just starting on their pregnancy journey.

The research discovered sperm ejaculated around three hours after a man has already climaxed are 'stronger' than those in the first round. They swim faster and are more likely to fertilise an egg due to a greater number of proteins in the sperm, Daily Mail reports.

Using sperm produced no more than 180 minutes after a man last climaxed can also boost IVF success rates by a third, the study found.

Dr Da Li, who ran the research project, says it contradicts the belief that it takes a man between 24 and 36 hours to 'replenish' his sperm bank after ejaculating. "For years, men have usually been advised to limit sexual activity to increase the chances of pregnancy," he told the Mirror.

"However, it's time to change our minds. Our data indicates couples with relatively normal semen parameters should have frequent sex around the ovulation period.

"This could make all the difference to their efforts to start a family."

Approximately one in six couples in New Zealand experience undefined infertility, and one in eight require some form of medical assistance. Fertility in both genders decreases with age - most rapidly in their 30s.