The chances of getting pregnant during ovulation depend on many factors. In the case of a healthy fertile woman coupled with a healthy fertile man, age is the strongest determining factor. Based on the probability calculations of Dr. Charles Westoff, fertility expert, Princeton University, here is a quick glance at the chances of getting pregnant during ovulation.
Early Twenties: In her early twenties, a healthy fertile woman has a 20%-25% probability of conception each month. The average time of conception for this age group is four months. The likelihood of conception within one year is 93%-97%.
Late Twenties: In her late twenties, a healthy fertile woman has a 15%-20% probability of conception each month. The average time of conception for this age group is 6.7 months. The likelihood of conception within one year is 86%.
Early Thirties: In her early thirties, a healthy fertile woman has a 10%-15% probability of conception each month. The average time of conception for this age group is 10 months. The likelihood of conception within one year is 72%.
Late Thirties: In her late thirties, a healthy fertile woman has a 9% probability of conception each month. The average time of conception for this age group is 12 months. The likelihood of conception within one year is 65%.
Source: Sherman J Siebler, MD, "How to Get Pregnant" New York, NY: Little, Brown and Co., 2009. Print.
When Are You Most Likely to Get Pregnant?
Intercourse is most likely to result in pregnancy when it's performed in the six day fertility window around the days of ovulation. According to clinical guidelines, a woman with regular periods experiences ovulation between days 10 and 17. One study that attempted to provide estimates on the likely occurrence of the six fertile days during the menstrual cycle discovered that only 30% of the participants fit the clinical guidelines. The study warns that the time of the fertile window can be highly unpredictable. 
Frequency of Intercourse
It goes without saying that unless a woman is being artificially inseminated or having in vitro fertilization, she cannot get pregnant unless she is having sex with a fertile male partner. If you are trying to get pregnant, physicians recommend that you have unprotected sex every other day or every third day to maximize your chances, but say that having sex every day or even multiple times a day during ovulation does not seem to increase the likelihood of pregnancy.  If you are a healthy woman under 35 who has been having unprotected sex for more than a year without a resulting pregnancy, it may be time to have your partner see a fertility specialist. 
Other Factors Affecting Fertility
Many lifestyle factors significantly impact fertility including alcohol intake, caffeine consumption, weight, smoking, and sexually transmitted diseases.
Alcohol. High levels of alcohol intake can reduce the chances of conceiving by up to 50 percent. 
Caffeine. Caffeine consumption affects both male and female fertility. Caffeine reduces likelihood of conception 10% for every cycle. Those that ingest more than 300 milligrams per day decrease chances of conception by 27% for each cycle. 
Weight. Being under or overweight can impact your chances of getting pregnant. Twelve percent of all infertility cases are linked to patient weight. Obesity accounts for six percent of primary infertility, while low body weight accounts for another six percent. More than 70% of women who are infertile due to weight issues are able to conceive once they resolve the problem. 
Smoking. Smoking not only reduces female fertility, but it also increases the likelihood of ectopic pregnancies and miscarriage. 
Sexually Transmitted Diseases. Both chlamydia and gonorrhea can reduce fertility. Chlamydia is often symptomless and when left untreated develops into pelvic inflammatory disease in 10-15% of women. The infections can cause permanent damage to the fallopian tubes, uterus, and surrounding tissue. 
 British Medical Journal, Wilcox, Allen J
The timing of the "fertile window" in the menstrual cycle: day specific estimates from a prospective study
2000, Volume: 321, No:7271, pages: 1259-1262
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