This is a topic where obsolete and unproven informations are paramount and can put significant stress on the love life of a couple trying to conceive. For example, it makes absolutely no sense for a woman to stay supine with elevated pelvis for half an hour after intercourse ... In the following text, I have tried to summarize the hard facts regarding natural fertility.
The fertile period in a woman's cycle is during the six days before ovulation. In this time frame, the couple should try to have intercourse about once every other day.
The maximum of fertility is reached on Day -2 (two days before ovulation). On the day of ovulation itself (day 0), fertility is quite low.
Hence ovulation test kits, testing for LH hormone in the urine, show positive only when the most successful day is already over, that is on day -1. I do explicitly discourage to time your love life with ovulation test kits.
Even in regular cycles of 28 days, ovulation can occur once on day 12 and only on day 16 in the subsequent cycle. There is simply no reliable method to predict the day of ovulation, except ultrasound to measure follicle size. This is why couples fare better with the recommendation to have intercourse twice per week, regardless of the phase of the menstrual cycle. This rule fits better the life pattern of working couples, and one avoids the dangerous tendency to have sex only when it «pays off».
Smoking is clearly and significantly detrimental (it can lower the success rates of IVF by up to 50% in both partners!). The influence of coffee and alcohol is less clear.
Overweight diminishes conception rates, even if the menstrual cycle is still regular. It increases the miscarriage rate and worsens sperm quality in men.
Age of the woman (over 35 years) remains the biggest and unsolved problem of reproductive medicine. Even assisted reproduction IVF and ICSI cannot compensate for female age!
It is a frustrating situation when fertility workup did not show any tangible cause in either partner. Those couples are by no means infertile, as 85% of couples conceive during the first year of trying, and half of the remaining couples during the second year. This is why one should not talk about «unexplained infertility», rather about «unexplained subfertility». Those couples have a reduced monthly fecundity, often for a reason yet unknown.
Couples with unexplained subfertility are being treated by ovulation induction and intrauterine inseminations, the second line being in vitro fertilisation (IVF).