For most of my life, my friends and I tried not to get pregnant. We discussed the pros and cons of the pill versus the vaginal ring, talked each other up before IUD insertions, and talked through whether to get the hormonal implant. I know women who have spent almost 20 years on hormonal contraception, focusing on school and careers and relationships during that time. But, around 30, a switch seemed to collectively flip in the women around me. The question became, instead of how do I avoid pregnancy at all costs — how do I get pregnant? And as soon as possible, because I spent all of this time getting my life in order and I’m ready now! Once I decided it was time, each month that was met by a period instead of a positive test felt like a small devastation. It took me almost a year longer to get pregnant than I expected, despite tracking ovulation with LH strips, charting cervical mucus, and timing intercourse to align with my most fertile days.
Most people know that fertility decreases with age. But by how much? And when does this typically happen? Is the cutoff linear? Or more of a cliff after a certain age? Fewer people are aware that birth control can cause a period of sub-optimal fertility after contraceptive use has stopped. Knowledge of how age and contraceptive use can affect how long it might take to get pregnant is crucial for people to effectively plan their lives. Someone might get off of hormonal birth control sooner than she wants to conceive, using non-hormonal methods in the meantime, to give her body time to adjust if she has accurate information about her fertility delay. Older women in particular might find this information valuable as they have more limited time to create the families that they want.
I created this calculator to provide people trying to conceive an estimate of how long they might expect to get pregnant based on recent high quality data. The data already exists. I hope to make this information accessible to everyone, empowering people to create the families and lives they want.
What studies does this calculator pull from?
The contraceptive delay data is from a prospective cohort study that pooled information from three separate studies of people trying to conceive. The studies are PRESTO (North America, 2013-19), Snart Gravid (Denmark, 2007-11), and Snart Foraeldre (Denmark, 2011-19). In total, the authors of this paper followed 17,954 women that had been trying to conceive for up to six cycles upon study start. The authors adjusted for a wide range of potential confounders, including age, BMI, smoking, income, intercourse frequency, and history of infertility. In this piece, I refer to this paper as the contraceptive study.
The age data also pulls from the PRESTO study (2013-2017), following 2,962 North American couples trying to conceive. Women were 21-45, not undergoing treatment for infertility, and had been trying to conceive for 3 cycles or fewer upon enrollment. Participants were tracked for up to 12 cycles. In this piece I refer to this paper as the age study.
What makes these studies high quality?
These studies are high quality primarily due to their size and that they are both prospective. Sample size is obvious — you can intuitively trust a study with almost 18,000 participants compared to one with 10. But why is the prospective component important? Gathering information from participants before and as they try to conceive eliminates recall bias. Memory is influenced by context. Take the example of a mother who found out her child has a birth difference. When asked to recall medication she took during pregnancy, she will likely think much harder and produce different answers than a woman whose child does not have this difference. Recall bias can confound questions of fertility for similar reasons — someone trying to get pregnant for a year likely pays close attention to all things fertility, more so than someone who got pregnant the month after they stopped using birth control. Our memories are more context-dependent than we give them credit for. Prospective studies remove this context-bias.
Mechanism for fertility decline with age
Unlike men, who produce sperm throughout their lives, women are born with a finite number of eggs (a fact that I love — the egg that became you was present in your mother when she was in your grandmother’s womb!) Over time, your egg supply naturally diminishes — not just through ovulation, but also a continuous process of follicular loss — until essentially zero eggs remain after menopause.
But egg quality typically affects fertility more than quantity. As eggs age inside the ovaries they accumulate chromosomal errors, enough so that by the age of 40 roughly half of embryos have chromosomal abnormalities that make a full-term pregnancy not viable.1
Beyond egg quality, the infrastructure that supports egg development and release breaks down with age. The hormonal coordination between the brain and ovaries can become less precise, leading to cycles without ovulation. These changes compound, leading to the acceleration in fertility decline in the late 30s and 40s.2
Mechanism for fertility delay after contraception
The pill, patch, and ring work by suppressing ovulation. While the hormones of these methods have a short half-life, meaning they do not stick around the body for very long, physical changes to the body that these hormones cause, like change in cervical mucus quality, which is integral for movement of sperm into the uterus, and endometrium thinning, can persist after contraceptive usage has stopped. Injectables also suppress ovulation, and do so via a deposit of progestin that is slowly released. Other progestin-only methods like the mini-pill and hormonal IUD don’t always suppress ovulation. Instead, they primarily function by thickening cervical mucus, making the cervical environment inhospitable to sperm. These effects can last after contraceptive use has ceased.
Important study takeaways:
No long term negative fertility effects from taking hormonal contraception
Some people think that hormonal contraception can harm fertility long-term. This belief likely originated in reaction to outdated methods of birth control. Older forms of the IUD (the Dalkon Shield specifically) were dangerous, causing bacterial infections that could damage fertility9 and hormone levels in older formulations of the pill were much higher as to increase the time to conception delay significantly.10 The contraceptive study shows that fertility is not damaged by hormonal contraception long term — participants who took the pill for 10 years had the same time to fertility outcomes as people who took the pill for just one year. Whatever the history of hormonal contraception, its long-term use does not damage fertility.
Fertility declines gradually then more dramatically after 37
There is a common cultural misconception that fertility is mostly steady until age 35, where it plummets, leaving a woman lucky if she can carry a child in her shrivelled womb. This is not what the data shows. Fertility is relatively stable from age 28-37, where it then declines more rapidly. But it is not a sudden “cliff” — few things in biology operate that way. A 38 year old woman who has never been pregnant, just got off of the pill, and is not tracking ovulation still has a 73% chance of conceiving within one year of trying. Those odds may be lower than some women are comfortable with, but the data shows that roughly three out of four women who want to get pregnant at this age will.
Higher rates of fertility for people who have been pregnant before
A surprising result from the age study is that the age-related fertility decline is weaker among women who have been pregnant before. The difference for women between the ages of 34-36 is particularly striking — the decline for women in this range who have been pregnant before is just 4% lower than the baseline, while it is 32% lower for women who haven’t. How could this be? Likely it is due to selection bias, or pulling from a subset of participants that is not representative of the entire group. Older women who haven’t been pregnant before from this group might be less fertile, while older women who have been pregnant before might be more fertile. Women who have previously conceived have demonstrated that their reproductive system works. This proven fertility may partially protect against the age-related decline, or at minimum means we know their fertility was sufficient to achieve pregnancy at least once. This is why the calculator asks whether you’ve been pregnant before — at 34 and older it meaningfully changes the estimate.
Male partner age had no measurable effect on fertility in this study
The age study found that fertility was not affected by the age of the male partner. This was true even after correcting for the age of the female partner. We do know that sperm quality declines with age — volume, motility, morphology, and DNA integrity have been shown to measurably decrease. But this study shows that, even with these non-optimal changes, fertility is not, on average, affected. How could this be? Likely due to the sheer number of sperm men produce. Men produce millions of sperm per ejaculate. Women typically release just one egg per cycle. If a large number of sperm are duds, the good ones can make up for those of poor quality. Quality of sperm appears to matter less than quality of the egg for this reason.
A caveat of this result is that there were only 37 men in the study over the age of 45. Male partner age not affecting fertility cannot be assumed true for men over the age of 45.
Rebound fertility
One of the strangest findings from the contraceptive study was that, for some hormonal contraceptives, fertility does not return to baseline after cessation of birth control. It actually rises above baseline levels. Called rebound fertility, fertility rises for some period between cessation of birth control and one year of trying to conceive.
The mechanism for this is not fully understood. Likely, though, it is an effect of the body recalibrating after a period of hormonal suppression — overshooting, if you will, before returning to baseline. This finding directly contradicts the widespread belief that hormonal contraceptives deplete or diminish fertility over time.
Caveats of the studies:
Study populations are not representative of larger societal demographics
The women that participated in these studies are a very white and very highly-educated group. 82.3% of women that completed the Wesselink study were college educated, and 80.8% were white. These numbers do not reflect the demographics of the United States, from which participants were drawn. Roughly 50% of women are white in the United States in the study age range3 while roughly 40% have a college degree.4 If you are white and have higher education, this study and therefore the numbers from this calculator reflect you well. If you are a person of color, in particular, the numbers are likely less accurate. Emerging work has shown that discrimination might affect fertility negatively.56 Additionally, the study participants were substantially more educated than the general population. Some research suggests fertility knowledge may be positively associated with education level (Pedro et al., 2018), though findings are mixed. It is plausible that more-educated participants are more likely to track ovulation and time intercourse to optimize their chances — behaviors that, if enriched in this study population, could make the time-to-conception estimates somewhat optimistic for the general population.
Small samples for less common methods
There were fewer participants for the patch (286 cycles tracked), injectables (416 cycles) and the implant (686 cycles) than other contraceptive methods. Due to the lack of data, confidence intervals for these methods are very wide, meaning the estimates for return to conception for these methods are less precise than other methods and therefore should be taken with less confidence.
No data on timing of last injection
Injectable methods work by injections of a synthetic version of progestin given roughly every three months. The contraceptive study did not track when the last injection was given, only that the participant had stopped. This makes it hard to estimate how much hormone was likely still in the body when the participant started trying to conceive, again decreasing the prediction confidence for this method.
These studies measured pregnancy, not live birth
Both of these studies tracked pregnancy outcomes, not miscarriages. This difference is particularly important for older women. Miscarriage rates increase with age, and the difference in numbers between a pregnancy and live birth is larger for older women.8 If you are 37 or older, consider discussing your specific situation with a healthcare provider rather than relying solely on population-level estimates.
The shared PRESTO data limitation
Both studies partly draw from the same dataset — the PRESTO cohort. This means that the age and contraceptive effects determined by this calculator are not entirely independent. Ideally each study would use a unique dataset to avoid this. An optimal study would estimate both age and method effects simultaneously in a single integrated model, but that study doesn’t exist yet.
Less reliable estimates for older women
The age study had only 38 women aged 40-45, and only one of those women had never been pregnant before. This means estimates for older women, especially older women who have never been pregnant, are determined from very few data points. The gap between the estimates for women who have been pregnant before and those who haven’t is likely exaggerated at ages 40 and above. Use the calculator outputs for this age range as rough guidance rather than precise predictions.
How does the calculator work?
This calculator works by multiplying the likelihood of conception considering contraceptive history and considering age. These numbers are determined based on fertility compared to baseline. For the age data, the baseline is the 21-24 age group. The baseline for the contraceptive data is people using barrier methods of birth control.
The baseline chance of conception for a peak-fertility couple is assumed to be 20%, meaning the couple has a 20% chance of conception per cycle. This percentage is based on numerous studies,1112 but is just an estimate.
Likelihood of conception for couples diligently tracking ovulation and timing intercourse is likely higher.1311 There is an option in the calculator for users tracking ovulation using LH strips, basal body temperature, or cervical mucus monitoring and timing intercourse to their fertile window. This option changes the baseline conception probability to 24% per cycle. While this option increases conception likelihood across all ages, the tracking bonus is derived from studies of younger women and may overestimate the benefit at older ages.
An additional methodological choice worth explaining: the age study calculated fertility rates for participants in age brackets — 21-24, 25-27, etc — rather than for individual ages. Use of these numbers would have meant that there was an abrupt decline in fertility between some brackets for the calculator that likely does not represent biology. To better represent the true biology, the calculator uses a linear fit between the brackets to avoid the arbitrary jump between bracket ages.
What to do with this information?
If you know you would like to conceive by a certain date, this calculator can be helpful for planning, providing information specific to your contraceptive history. Someone who has been on the pill should likely assume fertility will be delayed one to three cycles before returning to baseline. Someone who has been on Depo-Provera should give themselves a significantly longer window. If conception during this in-between period is not wanted, barrier or natural methods of contraception might be a good idea.
This calculator combines the age and contraceptive data from these two studies to give you a personalized estimate. Enter your age, contraceptive history, and target conception date and it will estimate your current per-cycle conception probability, your cumulative probability of conceiving within 6 and 12 cycles, and the date by which you should consider stopping contraception to give yourself the best chance of conceiving by your target date. But a big disclaimer: these numbers are estimates based on population-level data. Individual fertility rates vary widely.
I built this calculator because I wished it had existed when I was trying to get pregnant. Trying to conceive can be surprisingly emotional, and, at least for me, one way to cope with intense emotions and uncertainty is finding information I can trust. The information gap between what the research says and what most people trying to conceive actually know is typically large. I hope this calculator can be a useful tool during an intensely exciting and emotionally-heavy time of life.