Fertility decreases with age in both men and women. Though men are still able to release active sperm (and thus fertilize a woman’s egg) well into old age, women are born with all the eggs they will ever have—about half a million. And for both men and women, the ticking time clock of fertility speeds up after age 35, when fertility begins declining for both sexes. 

Although every pregnancy is unique and each pregnancy may be different for the same woman, age can and does influence the physical aspects of pregnancy. Here’s what women can expect by the decade.

In Your Twenties

Our bodies are primed to produce babies in our twenties. Physically, we’re at the top of our game: the irregular cycles of our teen years have evened out; eggs are healthy and fresh. The body can handle the additional load on the back, bones, and muscles better than any other time. 

The risk of miscarriage is low—around 10 percent—as is the likelihood for having a child with Down syndrome, spina bifida, or other chromosomal birth defect. (At 25, your risk of having a baby with Down syndrome is one in 1,250. By 40, the odds are one in 100.) Plus, women in their twenties have the lowest occurrence of medical complications during pregnancy. 

However, being pregnant in your twenties does have its hazards: the risk of preeclampsia, pregnancy-induced hypertension, is high in women age 20 to 24. This condition usually occurs with a first pregnancy—and women in their early twenties are most likely experiencing their first pregnancy. Also, younger women have less-developed eating and health habits, so they have a greater chance of having a baby with a low birth weight. 

In Your Thirties

One in five women today wait to have their first child until after age 35, and despite the advancing age and waning fertility, your body is still capable of producing and carrying a healthy, bouncing baby. You may just have to be more patient with your conception timeline— you have about a 20 percent chance per month you’ll get pregnant, and women in their thirties typically need three to six months longer than younger mothers.

Women in their thirties are also at an increased risk for developing pregnancy-related health concerns such as gestational diabetes, high blood pressure, and placenta previa, (a condition where the placenta grows near the cervix, covering part or all of the uterine opening, and causes severe bleeding during delivery). Plus, chronic conditions, such as diabetes and high blood pressure, become most apparent in this decade, and many symptoms can complicate pregnancy.

Age 35 marks the official start of “high-risk” pregnancies—though it’s not as if the clock ticks to your 35th year and instantly things go downhill. Instead, doctors use this age as a good indication of when to start watching for and monitoring certain possibilities with potential pregnancies. 

At this age, your odds of having a baby with a chromosomal problem (such as Down syndrome) also increase—if you’re pregnant at 35, your baby has a one in 378 chance of having Down syndrome and a one in 192 chance of having another chromosomal abnormality. The risk of miscarriage in the last half of this decade is double what it was in your twenties.

In Your Forties

It’s not entirely uncommon for women to have babies well into their forties. In fact, between 2007 and 2009, the CDC says women aged 40 to 44 experienced a six percent increase in births while all other age groups saw a decline. Still, the risks associated with having advanced maternal age—a term you will likely hear frequently if you’re expecting past age 35—aren’t anything to be ignored, especially the later you go into this decade.

Fifty percent of women in their forties will experience infertility or difficulty conceiving—a fact that may be explained in part by this statistic: women over 40 have about a five percent chance of getting pregnant per month. This figure is true for natural conception as well as medically assisted pregnancies.

The health risks associated with being pregnant are about the same for a mother in this decade as they were in the late thirties: she’s at risk for preeclampsia, gestational diabetes, and placenta previa, among other conditions. The difference between the two decades is in the potential effects on the baby. The chances of having a baby with a chromosomal abnormality increase dramatically—the risk of Down syndrome is 1 in 106 for a 40 year old; 1 in 30 for a 45 year old. The rate of miscarriage goes up, as well—the risk of losing a baby is greater than 50 percent in women aged 45 and older.

Women in their forties also have a higher chance of carrying multiples than younger women, even without medical intervention. Shifting hormone levels may stimulate the body to release more than one egg at ovulation.

Despite these risks, if you’re in good health, don’t have any preexisting conditions that can complicate a pregnancy, and eat well throughout your pregnancy, your overall risk of having any complications isn’t much higher than a woman in her twenties or thirties.