You’ve probably heard the term “biological clock” used to describe the idea that, in people with ovaries, fertility potential is constantly ticking away as if it’s on a timer that will eventually reach zero. That ticking apparently gets louder and more urgent through your 20s and 30s, pushing you to get pregnant and have a baby. And then, so the story goes, something significant happens at the pivotal age of 35, when your chances of conceiving without assistance allegedly fall off a cliff, take a nosedive, or otherwise abide by whichever foreboding metaphor might come to mind. Except…that’s not really what happens.
As is usually the case with body-related things, the truth is so much more complicated than a pithy moniker like “biological clock” makes it seem. Unless something unusual and extreme happens, a person with ovaries won’t suddenly wake up on their 35th birthday unable to conceive. However, it is true that fertility declines over time and that fertility experts see 35 as something of a landmark age in this process. But why is that exactly? Here’s how fertility changes over time and what to keep in mind if you know you want to conceive at some point in the future.
Fertility is based on what’s known as ovarian reserve.
The term “ovarian reserve” generally describes how many eggs someone has available for ovulation, the quality of those eggs, and how well those eggs can interact with the hormones that control their development and ovulation, according to the American Society for Reproductive Medicine (ASRM). (Quick refresher: Ovulation is when an egg gets released from an ovary so it can potentially meet up with sperm and become fertilized. For a step-by-step explanation of that whole process, head right over here.)
It’s widely believed that people with ovaries are typically born with a whopping 1 to 2 million eggs, according to the American College of Obstetricians and Gynecologists (ACOG). That sounds like a hell of a lot. It is a hell of a lot. But most of those eggs don’t get a chance to burst forth from an ovary on a mission to potentially be fertilized.
By puberty (which is when ovulation starts), this egg number has already declined to 300,000 to 500,000, according to the ACOG. This is due to atresia, the ASRM explains, or the natural process through which follicles that house eggs in the ovaries degenerate and die.
Ultimately, of all those eggs you’re born with, “only approximately 400 will ovulate over the reproductive lifespan,” Pinar H. Kodaman, M.D., Ph.D., assistant professor in the division of reproductive endocrinology and infertility at Yale University School of Medicine, tells SELF. (This number will of course change if someone is on ovulation-suppressing birth control for long periods of time or if they have a health condition that impacts ovulation, like polycystic ovary syndrome).
Ovarian reserve is generally estimated to peak between a person’s late teens and late 20s, the ACOG explains. It then starts declining by age 30 (as does your fertility) partly because viable eggs are dying more quickly. But it’s not just the quantity of eggs you have that matters, it’s also the quality.
In addition to your number of viable eggs decreasing with age, as eggs get older, they’re more likely to have too many or too few chromosomes, the ASRM explains, which can make it harder to become pregnant and carry a pregnancy to term. “[With age], pregnancy rates decline [and]miscarriage rates increase,” Richard J. Paulson, M.D., division chief of reproductive endocrinology and infertility at the Keck School of Medicine of University of Southern California (USC), and director of USC Fertility clinic, tells SELF. Eventually, by the time you get to about age 45, the chances of conceiving without assistance are drastically lower than before, Dr. Paulson explains.
This is part of perimenopause, or the time when ovulation and menstruation happen less often, according to the Merck Manual. At this point, your menstrual cycle may get shorter, which means your periods might become irregular. It may still be possible to get pregnant naturally at that point, but it’ll be much more difficult. “With irregular cycles, the window of fertility is harder to predict,” Dr. Kodaman tells SELF.
Eventually, when you haven’t had a period for at least a year, you’re officially in menopause. The average age of menopause is 51, the ACOG says.
Since there’s a pretty wide gap between your mid-30s and when menopause sets in, what’s all the fertility-related commotion surrounding age 35?
There is some merit to treating 35 as a fertility landmark, but it’s not as scary as it sounds.
All of the experts SELF spoke to pointed out that this focus on fertility and age 35 isn’t baseless. This notion is founded in science.
“Declining fertility around the age of 35 is not just guesswork,” Joshua Johnson, Ph.D., assistant professor in the division of reproductive sciences at University of Colorado Denver, tells SELF.
As the ASRM notes in their committee opinion on optimizing natural fertility, it can be significantly harder for people with ovaries to get pregnant without assistance after age 35. However, this phenomenon has also been overstated to a potentially alarming point when there’s actually a ton of nuance involved.
In general, most people are still fertile and able to get pregnant naturally at age 35, but research shows that egg quantity and quality start to go down “more noticeably” around this age, Mary Ellen Pavone, M.D., associate professor in the division of reproductive endocrinology and infertility and director of in-vitro fertilization (IVF) at Northwestern University’s Feinberg School of Medicine, tells SELF. This can make it harder to get pregnant.
Note that she said “more noticeably.” As we mentioned above, declining fertility is a gradual process that starts around age 30, not one that’s non-existent until it kicks into overdrive at age 35. Plus, a ton of factors can influence fertility in different ways in different people of various ages, like health conditions, anatomical issues, and environmental influences we’ll discuss in some detail down below.
Figuring out the exact rate at which fertility declines by certain ages is actually pretty difficult, the experts explain. Landing on these figures would require a large number of couples having frequent, unprotected penis-in-vagina sex and reporting this activity honestly and accurately. That’s something researchers can’t easily coordinate. Instead, experts have concluded that fertility starts a steeper decline at 35 based on a variety of creative data, Dr. Johnson explains. Some of this thinking is based on studies of historical populations who didn’t use contraception for religious purposes or because it didn’t exist.
For instance, a 2014 review of six data sets in Human Reproduction examined women’s marriage and childbirth records from French parish registers between 1670 and 1830, combed through similar statistics from the Netherlands in the 1800s, and also looked at four other populations that haven’t had access to birth control. In total, they collected data for 58,051 women who’d gotten married at some point before 1900 and had children.
Across the data, the scientists looked at women’s ages the last time they gave birth to help estimate the end of their fertility. (Remember, the people in these studies weren’t using birth control. The age at which they stopped having kids was a pretty good indicator of their fertility, though not an absolute one, which we’ll discuss below.)
The researchers found that, on average, less than 3 percent of women had already had their last birth at age 20, meaning nearly 98 percent of people studied were able to have babies after that. Around 96 percent could still give birth after age 25, 93 percent after age 30, and 88 percent after age 35. The numbers changed a bit more sharply after that; by age 38, 20 percent of people had given their last birth, meaning around 80 percent (still a big number!) had at least one child after that. At age 41, the number of people who had no more children increased to almost 50 percent, and it was at nearly 100 percent by age 50.
Overall, those are pretty encouraging findings in the face of rumors that fertility free falls after 35. There are some questions that hang over this data, though. For example, the researchers couldn’t determine how often the people involved were having sex and whether this decreased over time, which could have made them less likely to get pregnant at an older age. But as the study authors note, the fact that this data came from a large sample size encompassing multiple countries in different time periods and with “remarkably similar” ages at last birth suggests a broad pattern in decreasing fertility with age. With that said, it’s worth noting that this is an observational study based on data from a pretty long time ago. Although the experts note that it’s unlikely for such a central process to humanity to change wildly in that span of time, they also explain that behavioral, societal, and historical shifts certainly have happened since then and could influence these results.
Large clinical studies using data from assisted reproductive technology (ART) clinics have also helped experts parse out how fertility declines with age, Dr. Johnson says. A different Human Reproduction study, this one published in 2011, looked at data from 3,889 cycles of IVF or ICSI (intracytoplasmic sperm injection) in people who were receiving donor eggs fertilized with the recipients’ partner’s sperm. Specifically, the researchers looked at the egg donors’ ages, recipients’ ages, and recipients’ partners’ age to see which most impacted the likelihood of pregnancy.
The results showed that the partners’ and recipients’ ages didn’t significantly affect the chances of conceiving, but the egg donors’ ages did. (Fertility in people who produce semen doesn’t generally decline significantly until around age 50.) People who used eggs from donors who were 35 to 39 were 14 percent less likely to get pregnant and 18 percent less likely to have a live delivery than people using egg donors who were 30 to 34. That’s definitely a decrease with age, but it’s not a stark drop-off right from 34 to 35—it’s a general reduction in odds across the age range of 30 to 39. As with the historical data study mentioned above, the decrease in fertility jumped much more after age 40. People using eggs from donors who were 40 and up were 42 percent less likely to get pregnant and 54 percent less likely to have a live delivery than people using egg donors who were 30 to 34.
So, no, fertility doesn’t do a straight vertical dive at age 35, but it does start to decline more rapidly.
Age isn’t the only factor that affects fertility.
Another issue with focusing the fertility conversation mostly on your 35th birthday is that it downplays the role that other factors can have on your fertility. In reality, there are other things that can cause problems conceiving sooner than many of us expect. Health conditions that affect the reproductive system are prime causes of this. For instance, polycystic ovary syndrome is the most common cause of infertility—defined as not getting pregnant within a year of trying—in people with ovaries, according to the Mayo Clinic. One of its hallmarks is irregular or absent ovulation, which can make it really difficult to get pregnant.
Conditions that damage or change important structures like the ovaries, fallopian tubes, or uterus can also make it harder than usual to conceive. Endometriosis, for example, can cause lesions on all of these structures, along with inflammation and scarring that can impede normal fertilization. Untreated pelvic inflammatory disease, which happens when bacteria (typically from a sexually transmitted infection like gonorrhea or chlamydia) spread to the reproductive organs, can similarly cause scarring and abscesses that make it really difficult or impossible to get pregnant.
These types of reproductive conditions can make it harder to get pregnant at any age, but according to the ACOG, the chances of developing a disorder that affects fertility increase as you get older. That’s yet another reason why it can be harder to get pregnant with age.
Then there are other factors that can harm fertility in a young person, like radiation treatment near the ovaries, chemotherapy, and smoking. These can bring about a condition known as premature ovarian failure, which is when your ovaries stop working for some reason before age 40, the Mayo Clinic explains. Heavy alcohol use can also damage fertility before it would normally drop with age.
If you know you want to conceive in the future, it’s a good idea to plan ahead.
If your 35th birthday is tomorrow, there’s no real reason to treat it as a funeral for your fertility. The data and the various factors affecting your fertility are much more complex than that. That said, if you know that you want to conceive with your own eggs and you have the resources available, it’s a good idea to be proactive. That means doing things like checking in with your gynecologist, quitting smoking, and cutting back on alcohol if you drink heavily. And, you know, factoring in the sperm, too. For some, that might mean having regular sex once you’re actually ready to get pregnant. (The Mayo Clinic notes that the highest pregnancy rates happen in couples who have sex every day or every other day, though it really matters most in the three days leading up to ovulation and the actual day you ovulate since that’s when you’re most fertile.) For others, that might mean talking to an assisted reproductive technology specialist about your options for sperm donation if that’s a route you’re looking to go down. It all depends on your situation.
If you’re under 35, experts generally recommend that couples with a penis and a vagina between them try to conceive the old-fashioned way for a year before checking in with a doctor about potential testing for infertility, the Mayo Clinic says. In this type of partnership, infertility happens due to issues with the partner with the ovaries one third of the time, the partner with the testicles another third, and both partners the remaining third. Testing may help you find out where you fall.
If you’re between 35 and 40, that recommendation to seek fertility testing drops to six months, and if you’re over 40, you may want to consider testing right off the bat.
However, Dr. Paulson says, if you’re in any age range and worried about your chances of getting pregnant, there’s no harm in talking to your doctor earlier on in the process for peace of mind or an action plan.
In addition, if you have a specific reason to think that you could have trouble conceiving, you might want to see a professional sooner than a year into trying, Lusine Aghajanova, M.D., Ph.D., clinical assistant professor of obstetrics and gynecology in the division of reproductive endocrinology and infertility at Stanford University, tells SELF.
If it turns out that you are having or would have trouble conceiving for a medical reason, you and your doctor can discuss potential options, like treating any related health conditions you may have, egg or embryo freezing, intrauterine insemination, and IVF.
The bottom line
Yes, you are statistically less likely to get pregnant after the age of 35 than you are in your 20s and early 30s, but it’s not as if every person’s chances of getting pregnant immediately plummet the same way and at the same exact age. In lieu of more perfect data, what we have is “just an increased risk [of trouble getting pregnant], it’s not an absolute,” Dr. Paulson says.
The earlier you start trying, the better your odds of not only conceiving but maintaining a pregnancy and having a healthy baby, but also, that doesn’t take your life into account. There are tons of reasons—financial, emotional, relationship-related, and more—why you might not be ready to have a baby in your 20s and early 30s. Now that you know it’s a myth that your biological clock is racing against age 35, hopefully you feel like you have a little more freedom to have babies in your own time.