By age 40, women have a 44% chance of conceiving within a year.
When does a woman's fertility begin to decline?
The answer, experts say, is a cruel irony:
"When women are biologically most ready and able to have a baby, most women are not ready, emotionally, psychologically, socially," said Stacey Colino, a National Geographic contributing author who explored the factors that play into a woman's odds of conceiving.
Age delivers the harshest blow to a woman's fertility because she is born with all the eggs she will ever have.
"It's so unfair," Colino said. "For men, throughout their lives, the sperm production cycle is essentially reset every 72 days."
Fertility peaks between the late teens and 20s, when women have an 85% chance of conceiving within a year. By age 30, that percentage drops to 75. At 40, it's sunk to 44%.
"Aging eggs are not your friend," she said. "As you get older, your eggs are getting older too, and you're losing more of them."
But a woman's date of birth isn't the only determinant: factors may be genetic, hormonal, environmental or lifestyle.
"Smoking is really toxic to eggs. Heavy drinking can be. Certain drug use. Obesity is a problem when it comes to egg health," Colino said.
And many don't even know this. A recent survey found one in five women between the ages of 18 and 40 in the United States were unaware of the effects of age on their fertility.
"A lot of the experts that I interviewed for the article also said that women that they have worked with and treated were blindsided and had no idea that their fertility was at such risk," Colino said.
She hopes greater efforts are made to educate women on fertility -- years before they try to get pregnant.
"We could actually make better-informed decisions, maybe, or take steps to preserve our fertility," she added.
Colino reported that even pesticides and chemicals in plastics can impact egg quality. Learn more at NatGeo.com/health.
The Walt Disney Co. is the parent company of National Geographic Partners and this ABC station.