One of my former colleagues became pregnant her first shot out of the barn, the first month off the pill. Her story would hardly be noteworthy, except that she was 41 at the time.
She wanted to tell other women about her experience, she confided to me. She saw it as a sign that women can have children after age 40.
I simply nodded in response, while I privately wondered if she had not just been very lucky.
But–and this is key–how lucky?
Having a baby in your 30s and early 40s–and earlier, for that matter–is always a chance event. There will be outliers. Some women will give birth naturally at 44. Some women will suffer from early menopause at age 30. But outliers tells us little about the norm.
Anyone who wants to play the conception game, especially if they are postponing childbearing, needs to put anecdotes aside and try to grasp the actual odds. Here’s what every woman needs to know:
1) A woman in her late 30s, on average, has about a 20-25% chance of conceiving per cycle, according to a study led by statistician David Dunson of Duke University.
Dunson followed 768 Danish couples who were using natural methods to avoid becoming pregnant. Natural methods are imperfect, though: 433 couples became pregnant during the course of his study.
Oddly enough, that these couples were trying to avoid pregnancy was an advantage for Dunson. In every menstrual cycle, there is only a 6-day window–which opens five before ovulation–in which pregnancy can occur. The day after ovulation, the window has closed and will remain so until the next cycle. Each time a couple has sex within their fertile window–rolls their fertility dice–boosts their chances of conceiving. A couple who has sex multiple times within their fertile window effectively rolls the dice multiple times, raising their total chances of a winner.
This fact is a boon for couples trying to conceive, but a statistical nightmare for researchers. It’s very difficult to determine why some couples conceive while others do not. Are these couples truly more fertile or simply more active in the sack?
Luckily for Dunson, his couples were trying to avoid becoming pregnant, so few of them had sex on multiple days within their fertile window. The ones that inadvertently rolled their fertility dice, typically did so only once per cycle. And this allowed Dunson to estimate the chances of conception based on when couples had sex and the age of the woman:
As you can see, for most days within the fertile window, a woman in her late 30s has a nearly 25% chance of conceiving. But her overall chances are still considerably lower than those of a woman in her early 20s, mainly because she has lower chances of conceiving on her most fertile day. On her most fertile day–typically one to two days before ovulation–a woman in her late 30s has only about half the chances of conceiving of a woman in her early 20s. Her peak chances are about the same as those of a woman in her early 30s who has sex 3-4 days before she ovulates.
And this is why (2) properly timing intercourse to 1-2 days before ovulation is especially important for women in their late 30s.
Consider the findings of another prospective study led by Kenneth Rothman of Boston University. Rothman followed 2,820 Danish couples who were trying to conceive for up to a year. Overall, 72% of the 35-to-40-year old women conceived within a year of trying, lower than the 87% of 30-to-34-year-old women. But the chances were much higher for couples who timed intercourse: 78% of 35-to-40-year-old women conceived within 12 cycles, compared with 90% of 30-to-34-year-olds.
These percentages are consistent with other large studies on time to pregnancy:
(3) Most studies find that women in their late 30s have about a 70-80% chance of conceiving in a year. An additional 10-15% of these women will conceive within their second year of trying. Overall, about 90% of women in their late 30s will conceive within 2 years of trying.
In a survey of 1,976 pregnant British woman, led Mohamed Hassan and Stephen Killick, 72% of 35-to-39-year-old women reported conceiving within a year of trying, compared to about 90% of women under 25. The women in their late 30s also took much longer than younger women to become pregnant. Women in their late 30s, on average, conceived after 10 to 11 months of trying, compared with 5 to 6 months for women in their early 20s. Worse, these averages almost surely understate the true time to pregnancy, because Hassan and Killick only interviewed women who were already pregnant; they did not follow couples who were trying to conceive.
So far we have only examined women’s chances of becoming pregnant. But not all pregnancies result in live births. Even for women in their 20s and early 30s, about 10-15% of pregnancies will be lost before 20 weeks–that is, they will end in miscarriage.
(4) After a woman turns 37 or 38, her risk of miscarriage begins to climb steeply, reaching over 60% by age 45.
The best data on miscarriage rates comes from women undergoing in vitro fertilization (IVF), whose pregnancies are closely tracked from the time of conception. Among U.S. women undergoing IVF in 2010, the rate of miscarriage at age 40 was 28%, and by age 44, topped 60%.
Correspondingly, the per-cycle chance of a live birth using IVF drops dramatically with age, from 32% for 35-to-37-year-olds, to 22% for 38-to-40-year-olds, to 12% for 41-to-42-year-olds. For women over 44, the per-cycle chances of a live birth dwindle to 1%.
Most miscarriages occur in the first trimester; only 2% of pregnancy losses occur after 20 weeks. The most common cause of these early losses is chromosomal abnormalities. On average, a woman in her early 20s will have chromosomal abnormalities in about 17% of her eggs; this percentage jumps to nearly 80% by a woman’s early 40s. This is why the risk of Down’s syndrome and other chromosomal disorders rises with age, from less than 1 in 900 in at 30, to 1 in 180 at 35, to 1 in 60 at age 40.
This increase in chromosomal abnormalities in her eggs accounts for almost all of a woman’s diminished fertility in her late 30s and early 40s. The rate of egg loss does accelerates when a woman reaches 37 to 38, as does the decline in antral follicle counts (a marker of ovarian reserve). But it is the quality, not the quantity, of a woman’s remaining eggs that presents the primary constraint on childbearing during the late 30s and early 40s.
(As an aside, scary news articles claiming that “women have lost 90 percent of their eggs by the time they are 30 years old” are incredibly misleading. At birth a baby girl has only about a fifth of the eggs she had at 20 weeks gestation: about 1 to 2 million, down from a peak of 6 to 7 million. And by puberty, she will have less than half of the eggs she had at birth, between 300,000 to 500,000. So, if 30-year-old women need to race to have a baby because of their dwindling egg supply, then, by the same token, so would teenage girls. After all, teenage girls have have less than 85% of their initial egg supply remaining.)
Once pregnant, however, women remain capable of carrying a pregnancy to term until well into their 40s. Older women who undergo IVF using donor eggs have live birth rates comparable to rates of women in their 20s. Similarly, IVF success rates for 40-to-43-year-old women rise substantially when embryos are screened for chromosomal abnormalities before they are transferred, from 15-19% for untested embryos to 45% for tested embryos, according to one recent study. (This should not be taken as reassurance about IVF success rates in the early 40s, as these numbers assume that chromosomally normal eggs can be retrieved. But it does provide a compelling argument for freezing your eggs in your 20s or early 30s.)
Despite the substantial climb in miscarriage rates that starts around 37 or 38, many research studies lump together 35-to-39-year-olds. This grouping is arbitrary, made not because 35-to-39-year-olds are so similar in their fertility, but because 35 is the midpoint between 30 and 40. Thus, these studies will understate the ability of women in their mid-30s to have a baby, and overstate the ability of women in their late-30s to have a baby.
So far we have talked about the average. Even at young ages, though, there is a wide range in couples’ fertility. Dunson, for example, found an enormous range in the estimated ability to conceive, even for couples still in their 20s. Some couples had a more than 80% chance of conceiving each month, others a less than 5% chance.
What accounts for this wide range? A combination of factors, some related to lifestyle, and some related to luck. Smoking, drinking, genetics, exposures to pollutants in the womb, male fertility problems, and sexually transmitted infections like chlamydia all likely contribute.
Below, I briefly describe some of the key factors affecting women’s fertility in their late 30s.
(5) Women who have already given birth have higher chances of giving birth later in life. Women who have given birth before do not show much of a drop in fertility until about 38, whereas for women who have not given birth before, the drop comes earlier, starting around 33 or 34. (I discuss this phenomenon in detail here.)
This was true, for example, among the Danish couples in Rothman’s study. Couples with a prior birth had higher chances of conceiving overall, and showed a later decline in fertility:
Why might this be? There are two primary explanations, and both may play a role. The first is the idea that birth somehow “resets” a woman’s fertility, although the biological mechanism for this is unclear.
The second is selection bias: as a group, couples who have given birth before tend to be more fertile than couples who have not. This is because, during their 20s and 30s, highly fertile couples are more likely to become pregnant, either quickly or by accident, and sort themselves into the group with children.
(6) Your partner’s age matters too, particularly if he is over 40. Women in their late 30s paired with older partners have a lower per-cycle chances of conceiving and a lower chance of becoming pregnant within a year. In Dunson’s research, a woman in her late 30s with a partner of roughly the same age had an estimated 29% chance of conceiving on her most fertile day; a woman in her late 30s with a partner 5 or more years older had only a 18% chance of conceiving on her most fertile day.
In contrast, having an older partner did not significantly affect a younger woman’s chances of conceiving, at least in Dunson’s research. This may have been because, in Dunson’s research, the partners of women under 35 tended to be fairly young. For women in their early 30s, for example, the average age of their partners was 33.
By contrast, in Hassan and Killick’s survey of nearly 2,000 British women, women whose partners were over 45 showed a dramatic increase in time to pregnancy: they took an average of 19 months to conceive. And women whose partners were over 50 took twice as long: an average of 32 months to conceive. In Rothman’s study of Danish couples trying to conceive, there was a significant drop in couple’s conception rates when the man reached his late 30s. And women in their late 30s with partners over 40 face roughly double the odds of miscarriage compared to women with partners of the same age.
Smoking in particular has a huge effect on women’s fertility. Carcinogens in cigarette smoke are found in follicular fluid and appear to directly damage ovarian follicles. Smokers go through menopause an average of 1-2 years earlier than non-smokers. Women of all ages who smoke have about double the chances of remaining childless after trying to conceive for 5 years (10% versus 5%, according to a recent large study). The same is true for couples undergoing assisted reproduction. Smokers have much lower IVF success rates. And smoking during pregnancy raises the risk of miscarriage by about 30%. Fortunately, many of smoking’s ill effects are short-term and therefore reversible. Former smokers’ chances of conceiving are nearly as high as those who have never smoked.
Not smoking and otherwise maintaining a healthy lifestyle are especially important for women trying to conceive in their mid to late 30s. In Rothman’s study of Danish couples, for example, women in their late 20s or early 30s did not differ from 20-to-24- year-olds in their ability to get pregnant. Women in their late 30s, on the other hand, had about three-quarters the fertility of the 20-to-24-year-olds, but only in adjusted analyses.
What does this mean? Basically, the fertility of the 35-to-40-year olds looked about the same as that of the 20-to-24-year-olds, until Rothman controlled for other differences between the two groups. The lifestyle of the 35-to-40-year-olds boosted their odds of conceiving: they were less likely to smoke, drink heavily, be overweight, or have recently used a hormonal form of birth control (going off the pill causes a temporary drop in fertility), and more likely to time intercourse than were the younger women.
The Bottom Line
At the end of the day, there will always be some risk of being unable to have a child. Human reproduction is a chancy business. There are no guarantees, not even for women who start trying to conceive in their early 20s. Still, the earlier you start trying, the higher odds you have at the outset, and, just as critically, the longer you have to play those odds.
So while in a sense there are no statistics for the individual–a woman will end up with a baby or she won’t; her personal probability is always 0 or 1, not .25 or .4–understanding the odds can empower women to make informed choices.
Today this is more important than ever: recent advances in reproductive technologies, like egg freezing and embryo freezing can allow women to slow down reproductive time, especially if they are proactive. Here I review the pros and cons of egg freezing and how success rates are affected by the woman’s age at freezing.
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