The study published by the University of St. Andrews and Edinburgh University in Scotland found that women have lost 90 percent of their eggs by the time they are 30 years old, and only have about 3 percent remaining by the time they are 40.
It's common knowledge that women have more difficulty conceiving as they age, but this is the very first study believed to quantify the number of eggs lost and it shows that the decline is more rapid than previously believed.
Over time, the quality of ovarian eggs also deteriorates, increasing the difficulty of conception and the risk of having an unhealthy baby.
The study was based on information collected from 325 women of varying ages in the United Kingdom, the United States and Europe.
Dr. Marie Savard, "Good Morning America" medical contributor, visited "GMA" to discuss the issue and its implications for moms-to-be.
"Women lose eggs a lot faster than we thought," she said. As you get older, conceiving is "much more difficult. ...Even all those assisted reproductive techniques are challenges."
"That biological clock does tick," she said, adding that her advice to women who want kids is, "the sooner the better."
Your Fertility Questions Answered
Q: Is there anything you can do to slow down the loss of fertility?
A: You can't reverse the biological clock, but Savard said there are certain factors within your control that have an impact upon your fertility.
For example, stopping smoking, keeping your weight down and controlling stress can all slow the loss of fertility.
Women who have more pregnancies are fertile for longer, and some women are born with more eggs than others, she said. The more eggs with which a woman is born, the longer she will be fertile and more time she will have until the onset of menopause.
Falling Fertility: When to Bear Children
Q: If someone takes the pill, does that affect her fertility?
A: No. Savard says birth control pills do not in any way affect a woman's fertility or egg count.
"Even if you'ree on the pill for a long period of time, that doesn't influence your fertility," she said.
The length of a woman's cycle -- whether it is longer or shorter -- does not predict a woman's fertility, she added.
Q: Should women have children earlier?
A: Savard said women should have children earlier, if possible. Healthy women in their late 30s and early 40s who think they can postpone pregnancy may be jeopardizing their chance of conceiving, she said.
Women who are considering pregnancy should talk to their doctors about their family history.
Savard noted that technology to aid conception is much less successful as women age. Only 10 percent of women aged 40 will have a successful pregnancy with a single attempt through in vitro fertilization.
Q: Is there a way for women to find out what their egg count is?
A: There's no way to determine an exact number of eggs, Savard said. There are indirect measures – including ultrasound to measure the size and volume in the ovaries -- as well as blood tests to check for hormone levels.
At the end of the day, Savard said it is important for women to recognize that "time does matter."
"If you're planning a family, don't wait so long," she said. "It's life, it's the truth -- we have all the eggs we're ever going to have and we'd better take good care of them."