Age & Fertility
It is well documented that fertility declines with aging - for both male and female partners. The primary reason for this is that as men and women age, they produce fewer high quality gametes (sperm in men, eggs in women). While processes vary between men and women, the end result is the same.
At birth, the ovaries contain all the eggs a woman will ever have. By the onset of puberty, this number has already dropped into the 100,000's. For most women, this means there are plenty of eggs to last from menarche to menopause. For other women, those with low ovarian reserve or premature ovarian failure, supply is limited and fertility declines much sooner. While there is no standard age that a woman's fertility begins to diminish, population research has revealed that, as a whole, female fertility begins to decline sharply around the age of 35. By age 40, a woman's chance of conceiving within one year is roughly half that of what it was when she was in her late 20's. With every year after 40, the chances of conceiving naturally continue to decline sharply, with women over the age of 44 having less than a 2% chance of conceiving using their own eggs. For a general chart of how age influences fertility, see here.
Conception is not the only difficulty that increases with age. Miscarriage rates climb sharply as women enter their 40's, up to 50% from about 10% for women in their 20's. Pregnancy complications can increase as well, especially factors like diabetes and heart disease. This risk of genetic problems increase with age. At age 40, risk of carrying a child with Down's syndrome is about 1 in 100. At age 45, it's about 1 in 30.
While complications do increase for older women trying to conceive, it's not impossible, and many women in their 40's will go on to have happy, healthy babies. It's important to remember that everyone ages at a different rate - the best way to determine your reproductive age is through ovarian reserve testing. If your ovarian reserve has already declined, there are many options for improving your chance of motherhood including IVF, egg donation, gestational surrogates, natural remedies like acupuncture or supplements, and adoption. For younger women who are postponing having a family, oocyte retrieval and preservation may greatly increase your chances of conceiving later in life.
Male fertility is also impacted by age. In addition to the loss of sexual function, male gamete production also decreases with age. A 2006 study published in Fertility and Sterility (Killick, et al) found that, on average, it took men over 45 five times longer to impregnate their female partner as men under 25. While men continue to produce sperm throughout their lives, quality and quantity tend to decrease with age, partially due to a decrease in testosterone associated with aging. Between the ages of 30 and 50, important sperm health parameters such motility and morphology may decline by up to 50%. These measures are important because they demonstrate the genetic health of the sperm. Older men, with fewer healthy sperm, are not only more unlikely to help their partner conceive, but they are more likely to contribute to genetic abnormalities in the fetus. We are still learning exactly how men's role in passing along important genetic information works, but new evidence definitely suggests that younger male partners also produce healthier offspring. The bottom line: even though men may be fertile well into their 60's, decreasing rates of conception and increasing rates of genetic abnormalities can have pronounced effects on fertility outcomes.