Caffeine and Fertility: The 1988 Coffee Study, Revisited

The advice on caffeine and fertility usually arrives in one line, and the line is usually "cut it out." A nurse says it at the first appointment. An app slips it onto a preconception checklist between folic acid and more sleep. A friend who got pregnant last spring tells you she quit coffee the week she started trying, and you can hear in her voice that she thinks it mattered.

I've spent years around ceremony and healing, from the Putumayo in southern Colombia to the Maya side of Mexico, where I live now. Every tradition I've sat with has rules. I've learned to treat a health rule the way I treat a ritual somebody hands me: with respect, and with three questions. Who made it? What did they measure? And what happened when somebody else checked?

Ask those three questions about caffeine and fertility and the one-liner falls apart, then puts itself back together in a different shape. The rule started with a study of 104 women that found coffee halved the odds of conceiving. Thirty years of bigger studies mostly didn't find that. They found something else, in two places almost nobody looks. One is what happens after the test turns positive. The other is his cup.

Where the caffeine and fertility rule came from

In December 1988, Allen Wilcox, Clarice Weinberg and Donna Baird of the US National Institute of Environmental Health Sciences published a short paper in The Lancet. They had been following women in a larger study of early pregnancy loss, and caffeine and fertility was a side question. They took the 104 healthy women who hadn't conceived in their first three months of trying and interviewed them about coffee, tea, caffeinated sodas, alcohol and cigarettes.

Open bound journal from the 1980s on a library table with a coffee cup pushed to the edge — The rule began with 104 women and one interview. The paper said so itself

The dividing line was the median intake, 3,150 mg of caffeine a month. That's about 105 mg a day, roughly one mug of brewed coffee. Women above it were about half as likely to conceive in any given cycle as women below it. The more caffeine, the lower the odds: in the heaviest group, the per-cycle chance fell to about a quarter of the lightest group's. Thirteen cycles in, 28% of the high-caffeine women still weren't pregnant, against 6% of the rest.

One mug a day. That describes most adults I know, which is exactly why the paper became a rule about caffeine and fertility that people still repeat almost forty years later.

Look at what that caffeine and fertility study actually was, though. A hundred and four women, every one of whom had already spent three months not conceiving. Caffeine asked about in a single interview. And a closing admission from the authors that they had no idea how caffeine could do this. It was a small, honest paper that flagged its own limits. The headline travelled. The limits stayed in the journal.

What thirty years of caffeine and fertility research found

In 2017, Julie Lyngsø and colleagues at Aarhus University in Denmark pooled the research on caffeine and fertility in a dose-response meta-analysis for the journal Clinical Epidemiology. They looked at two kinds of outcome. One was natural fertility: the chance of conceiving per cycle, or how long it takes. The other was the result of fertility treatment.

For natural fertility they found no clear association with coffee or caffeine. For couples in treatment, based on only two studies, no association with pregnancy or live birth either. Their own summary was that caffeine and fertility, in the sense of getting pregnant, seem to have little if any connection.

The 1988 halving didn't come back in the bigger caffeine and fertility data.

Something else did. In the same analysis, caffeine was linked to miscarriage, and the risk climbed with the dose. At 300 mg a day the relative risk was 1.37, about a third higher. At 600 mg it was 2.32, more than double. So the caffeine and fertility question has moved. It's less about whether you can conceive with coffee in your life and more about what caffeine does once you have.

The caffeine and fertility limit may be set too high

The Danish authors went one step further than most reviews do. Health bodies such as the European Food Safety Authority and the WHO advise a ceiling of two to three cups, 200 to 300 mg a day. The authors wrote that this ceiling may be too high. Their worry was the loss signal, which in the pooled data starts below that official ceiling.

The caffeine and fertility question nobody asks him

Almost every conversation about caffeine and fertility is aimed at the woman. The data aren't.

In 2016, Amelia Wesselink and colleagues at Boston University reported on 2,135 people planning a pregnancy in PRESTO, a North American study that enrols couples before conception and follows them for up to a year. Caffeine was asked about at the start. On caffeine and fertility, the split between the partners was sharp. For the women, total caffeine intake wasn't associated with how quickly they conceived. For the men, it was. Men drinking 300 mg or more a day had a fecundability ratio of 0.72 compared with men under 100 mg: roughly 28% lower odds of conception per cycle. The pattern wasn't a neat straight line, and the authors said so. Caffeinated sodas and energy drinks stood out on his side. On hers, caffeinated tea showed a slight reduction.

Two empty energy drink cans and one unopened in a work van at 7:40 in the morning — Men at 300 mg a day or more: about 28% lower odds of conception per cycle in one North American study

The same year, Germaine Buck Louis and colleagues at the US National Institutes of Health published the LIFE Study in Fertility and Sterility. They recruited couples in Michigan and Texas before conception and had both partners log their caffeine, alcohol, cigarettes and vitamins every single day. Of the 344 women who became pregnant with one baby, 98 (28%) lost the pregnancy within the first seven weeks. More than two caffeinated drinks a day before conception was linked to a higher risk of loss for women (hazard ratio 1.74). For men, the number was almost identical: 1.73. That's the caffeine and fertility result I'd want every couple to hear.

Read that slowly if you're the one who brings two energy drinks to work. His caffeine and fertility don't show up on any preconception checklist I've seen. They show up in the data, sitting right next to hers. I wrote earlier this year that male infertility is half the story and gets a fraction of the attention. The coffee question is one more place where that's true.

Why nausea doesn't explain it away

There's a sensible objection to every caffeine and fertility study that looks at miscarriage. A healthy early pregnancy often brings nausea and food aversions, and coffee is one of the first things many women go off. So women whose pregnancies are going well drink less coffee, and coffee ends up looking guilty when it's really a bystander. Epidemiologists call this reverse causation. It's a real problem for any study that measures caffeine during pregnancy.

The LIFE team took the objection head on. Their link showed up in caffeine measured before conception, when nobody was pregnant yet and nobody was nauseous. They wrote that this finding fails to support reverse causation as the explanation. And to my mind the men's numbers make the same point even more plainly: his morning sickness isn't a thing. The authors were also clear that they couldn't rule out other hidden factors.

Now the part that argues the other way

This is where caffeine and fertility gets uncomfortable, because the counter-arguments come from both sides at once.

From one side, Jack James, a psychologist at Reykjavik University, went through two decades of studies on caffeine in pregnancy for a 2021 review in BMJ Evidence-Based Medicine. Of 42 sets of findings from 37 observational studies, 32 showed a significantly higher risk with caffeine. He found frequent reports of no threshold below which the association disappeared. His conclusion goes much further than any app: women who are pregnant or trying to become pregnant should be advised to avoid caffeine, full stop. If he's right, "moderate" is the wrong word to build caffeine and fertility advice around.

From the other side, the American College of Obstetricians and Gynecologists wrote in a 2010 committee opinion that moderate caffeine, under 200 mg a day, doesn't appear to be a major contributing factor in miscarriage or preterm birth, and that no final conclusion could be drawn about high intake. And every caffeine and fertility study in this post, including the ones I find most convincing, is observational. Nobody randomised couples to coffee or no coffee. The Danish review states plainly that residual confounding can't be excluded. Coffee travels with other things: cigarettes, long shifts, short nights, a body running on stress. A cup can be a marker for a life as much as a cause.

Here's where I land, and you're free to land somewhere else. The fear that a morning coffee stops you getting pregnant rests on one small study that the bigger ones didn't repeat. The link with pregnancy loss is more consistent, it shows up in his intake as well as hers, and it isn't proven. That combination doesn't call for panic. It calls for bringing both cups down, calmly, before you need to.

What the cup is doing for you

Nobody drinks four coffees a day for the taste of the fourth. Caffeine works by blocking adenosine, the chemical signal of building tiredness. It hides how tired you already are.

Trying to conceive is tiring in a way people don't talk about. The waiting, the tracking, the appointments, the nights you lie awake doing maths with dates. If you've been sleeping badly through all this, the coffee is quite possibly what's holding the day together. In the language I use about the body, it's part of the bracing. Your body isn't broken. It's braced, and caffeine is one of the ways it stays braced.

That matters for how you cut down. Pull the coffee out abruptly and you don't remove the tiredness. You meet it, usually with a headache, often in the middle of the two week wait, the least restful fortnight of the cycle. A rule about caffeine and fertility that ignores this turns into one more exam you can fail. I've written before about how stress and fertility interact; adding guilt over a latte to the pile doesn't help either.

How to handle caffeine and fertility without making it a test

Do it together. If one of you is cutting caffeine for fertility, both of you are. His sodas and energy drinks count.
Taper over a week or two rather than stopping the day you start trying. Swap one cup at a time.
Read the label or ask the café. A large takeaway coffee can hold far more caffeine than a mug at home, and tea, cola and energy drinks all add to the total.
Keep the ritual and change what's in it. Same cup, same chair, same ten minutes in the morning, with decaf or a herbal tea in it. Often the pause was the point.
If you're in the 90 days before conception, that's a sensible window to bring it down slowly, alongside sleep.
Once you're pregnant, ask your midwife or doctor which limit they use, and follow that over anything you read online, this post included.

Two mugs of herbal tea on a back step at dawn, a decaf morning ritual for two — Same cup, same step, same ten minutes. Different thing in the cup

When to stop reading and call your clinic

Caffeine and fertility is a small lever. It isn't a reason to wait longer before getting checked. If you're under 35 and have been trying for twelve months, or 35 or older and trying for six, see a fertility specialist, and make sure the first round of tests includes a semen analysis. Go sooner if your periods are irregular or absent, if there's a known history of endometriosis or pelvic infection, or after two or more losses. If you've already had repeated losses, recurrent miscarriage has its own work-up, and coffee is a small part of that conversation at most. In pregnancy, bleeding or severe pain goes to your doctor first.

When it comes to caffeine and fertility, the coffee you drink with your partner on a slow Sunday isn't a moral question. The fourth cup on a weekday might be worth a look. Have that look together.