Traditionally, husbands and wives live together while trying to conceive.
But for some couples, who are in long-distance relationships because of their careers, it may make more economic sense to wait until a baby is on the way and then move in together, says gynaecologist Vanadin Seifert-Klauss in Munich, Germany.
"The patients want to become pregnant first before then changing their lives," she said.
But it isn't that simple. The likelihood that a cohabitating couple who have sex regularly will achieve pregnancy within a year is 80 per cent if the woman is healthy.
For couples in a long-distance relationship, the likelihood is lower.
Even for couples who spend every weekend together, the chances of pregnancy are lower, said Klaus Doubek, a leading gynaecologist.
"Their time together must happen in the same window as ovulation," he said. But many women are still unsure when this happens, Seifert-Klauss said.
As a matter of fact, after ovulation has already occurred, the likelihood of becoming pregnant is close to zero. "The egg only remains ready for conception for about twelve hours," Seifert-Klauss said.
There is a higher likelihood of becoming pregnant when having sex two or three days before the ovum is released.
Doubek recommends women track their menstrual cycle in a calendar to help them determine the best opportunity. There are also urine tests available. Body temperature can be a good indicator of when a woman is most fertile.
"Women with a regular cycle can easily calculate when they will ovulate next and then discuss with their partner to ensure they are both free," he said.
There is often a lot of frustration when there is no pregnancy after the first few months of trying.
Melanie Henes, who works at a clinic that assists couples who want to have children, recommends a woman visit with a doctor if there is no success after the first several months.
A hormone test would allow a doctor to see if a woman has a thyroid issue or polycystic ovarian syndrome, a condition which affects 5 to 8 per cent of all women and can keep a woman from becoming pregnant.
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If ovulation doesn't occur at the same time a couple is together or if the distance is so great that they only meet one or two weeks a year, a gynaecologist can help.
One option is to trick the body by taking birth-control pills and stopping them suddenly.
"In some women, that leads to a surge of a substance in the blood, follicle stimulating hormone (FSH) and activates the ovaries, so they have a higher likelihood of ovulation." Seifert-Klauss said.
The second option is triggering ovulation at the gynaecologist's surgery. An ultrasound allows the doctor to see when the follicle - the envelope that holds the egg - is big enough to be injected with another hormone, human chorionic gonadotropin (HCG).
"The pregnancy hormone HCG then ensures that the follicle bursts and the ovum can be fertilized within 36 hours.