Whether you are trying to get pregnant soon or planning to wait a while, everyone should know the facts related to fertility.

Here are nine things you may not know about your fertility from the physicians at Pacific Fertility Center.

1) A woman's fertility is mostly determined by her eggs.

The number of eggs and their quality determine the chance of pregnancy. For a woman in her mid-20s, about one in four eggs will produce a healthy embryo. For a woman at age 40, about one in 20 eggs.

Doctors believe the number of eggs at birth is set with about a million eggs in the ovaries. Over a reproductive lifetime, a woman will ovulate 500 times. For every egg ovulated during your reproductive life, about 1,000 eggs undergo programmed cell death. Those that remain at older ages have much higher chromosome abnormality rates, so pregnancy rates decline with age.

2) Regular menstrual cycles are a sign of regular ovulation.

Most women have regular cycles lasting 27-29 days, a sign of regular ovulation. Women who don't ovulate regularly have irregular cycles. Most irregular cycles result from a problem in the hypothalamus and pituitary gland, the brain structures that control ovulation, though there are other causes.

3) Reproductive potential is primarily linked to a woman's age, not her lab results.

Female age is the best predictor of egg quality. Follicle-stimulating hormone (FSH), anti-Mullerian hormone (AMH), and estradiol are predictors of the number of eggs available. Women with abnormal lab test results can still conceive, especially when younger.

4) Most women with blocked fallopian tubes are unaware they may have had a prior pelvic infection.

About 10 percent of infertility cases are due to tubal disease. One major cause is a prior pelvic infection from a sexually transmitted disease. These infections cause few symptoms, so you may be completely unaware your tubes are affected.

5) Good fertility requires a good foundation.

Excellent nutrition, stress reduction, and a physically strong body are foundations for optimizing your fertility.

6) Fertility rates decline dramatically through age 40-50.

While fertility treatment can be successful, conception rates are low beyond age 43. Most women who conceive in their late 40s with fertility treatment are using donated eggs from younger women.

7) Having fathered a pregnancy in the past does not guarantee fertility.

Sperm counts can change quite a bit with time, so never assume that a prior pregnancy guarantees fertile sperm. Obtaining a semen analysis is the only way to confirm sperm are still healthy.

8) Vitamin D may improve results of fertility treatments.

A study from the University of Southern California suggested that women undergoing fertility treatments who had low vitamin D levels, might have lower conception rates. This vitamin is also essential during pregnancy. In Northern California there is essentially no natural Vitamin D production six months of the year. At Pacific Fertility Center, we recommend our patients check Vitamin D levels and take supplements if their levels are low.

9) Being underweight or overweight is linked with lowered fertility levels.

The evidence in recent years is that obesity is clearly linked to longer time to conception. A body mass index (BMI) less than 18 or over 32 is associated with problems ovulating, conceiving, and during pregnancy.

// Pacific Fertility Center, 55 San Francisco St., Ste. 500, pacificfertilitycenter.com