While there's disagreement about whether or not a man should refrain from sex and ejaculation for a period of time before his partner ovulates, recent research indicates a man's sperm are healthiest when he has had daily sex in the week leading up to attempted conception [source: von Radowitz].
It seems the constant renewal of sperm prevents the time-related DNA damage that can occur in sperm that spend too much time in the testes. Dr. David Greening from the Sydney IVF clinic conducted a study involving men who had a higher-than-usual rate of DNA damage in their sperm cells. His team found that the percentage of sperm with damaged DNA decreased following periods of daily sex. Though the actual sperm count was cut by half, the sperm that were present were stronger, healthier and more mobile. However, the sperm count dropped to less effective levels if couples had sex three times a day or more [source: von Radowitz].
The odds of successful fertilization increase if at least 40 percent of the sperm are moving [source: Mayo Clinic]. Not all sperm cells have similar structures — some have misshapen heads or damaged tails. In fact, many of the sperm are flawed in some way. Fortunately, even men with a low count of normal sperm can still father children, though it may take longer than it would if they a higher count [source: Trost].
For women, any abnormality in the reproductive system can cause major problems when it comes time to conceive. Damaged fallopian tubes or uterine issues can prevent conception or a successful pregnancy.
In addition to receiving regular medical checkups, women can improve the odds of conception by staying healthy, maintaining a proper weight, staying away from tobacco, caffeine, alcohol and drug use, and protecting themselves against sexually transmitted diseases.
To increase the odds of pregnancy, have sex at least every other day around the time of ovulation [source: Mayo Clinic]. Also, talk to your doctor to find out which lubricants may be adversely affecting the mobility — and eventual success — of the sperm.
Assuming both parties are healthy, how long does it take for fertilization to occur once sperm cells have entered the vagina? We'll find out in the next section.