Smoking, weight, alcohol and coffee: what holds up
Two of these have good evidence against them. Two are repeated far beyond what the evidence supports.
Lifestyle advice in fertility clinics tends to arrive as one undifferentiated list. The evidence behind the items on that list is very uneven.
| Factor | Effect on time to conception | Verdict |
|---|---|---|
| Smoking | Odds of infertility 1.60 (95% CI 1.34 to 1.91) in women smokers | Supported |
| Body mass index 30 to 34, and 35 and above | Fecundability ratio 0.75 and 0.61 against normal weight of 20 to 24 | Supported, and dose dependent |
| Alcohol under 14 servings a week | Fecundability ratios 0.97 to 1.01, no discernible effect | Not supported |
| Coffee, one to two cups a day | No clear association with conceiving | Not supported |
Smoking. Twelve studies pooled. The fertility societies add that menopause arrives on average one to four years earlier in smokers, and that miscarriage risk is higher in both natural and IVF pregnancies. They also note that the data do not conclusively show smoking lowers male fertility, which surprises people.
Weight. In 1,651 Danish women planning pregnancy, time to pregnancy lengthened progressively as body mass index rose above the normal range: fecundability ratio 0.83 at 25 to 29, 0.75 at 30 to 34, and 0.61 at 35 and above, against a normal weight of 20 to 24. Note that the first of those three intervals touches 1.00, so the overweight band on its own is not clearly separated from normal. A separate study of 3,029 subfertile couples found the chance of spontaneous pregnancy fell about 4% for each unit of body mass index above 29. What has not been shown is that losing weight before conceiving improves live birth. That claim is often made and this guide will not make it.
Alcohol. In 6,120 Danish women followed for up to twelve cycles, drinking under 14 servings a week had no measurable effect on how quickly they conceived. This is a different question from drinking in pregnancy, where no safe level has been established and abstinence is the advice. Both statements are true and they should not be run together.
Caffeine. For conceiving, no clear association. For miscarriage, the picture is different: at 300 mg a day the pooled relative risk was 1.37, and at 600 mg a day it was 2.32. Those are observational studies, and nausea in healthy pregnancies suppresses coffee intake, which biases this kind of study toward finding harm. Treat the conception claim as unsupported and the miscarriage association as real but uncertain in size.