How To Be Fertile

So you want a baby. How do you maximize your odds of getting pregnant?

The always excellent Expecting Science has covered the issue of maternal age very well; in short, you’re still pretty fertile in your early 30’s, the big dropoff in fertility begins around age 37, and women in their late 30’s still have a 70-80% chance of conceiving within a year.

In other words, DON’T PANIC.  Also, have sex more frequently, especially around the fertility peak two days before ovulation.  A 35-year-old woman having sex 2 days before ovulation has a higher chance of getting pregnant than a 25-year-old woman having sex 3 days before ovulation; timing trumps age.

Apart from age, what are the main risk factors for infertility, and how can you reduce them?  (Infertility is defined as trying to get pregnant unsuccessfully for at least a year.)

1. STDs and their sequelae. A history of STDs multiplies your risk of infertility by 4x, and history of pelvic inflammatory disease (which can result from untreated gonorrhea or chlamydia) multiplies your risk by 45x.[1]

2.  Varicocele. A common (15-20% of men) abnormality in the testicular veins called a varicocele is a leading cause of male infertility, increasing the risk of infertility by a factor of 28x.[1]  It’s often noticeable through a visible or palpable enlargement in the testicle, and a feeling of heaviness or pain. Varicoceles can be detected with ultrasound and treated with surgery.

3. PCOS.  Polycystic ovary syndrome, a condition in which women don’t ovulate due to elevated male hormone levels, is a leading cause of female infertility, affecting 5-10% of reproductive-age women.  Risk factors for PCOS include obesity and genetic factors; it can be treated with weight loss and metformin. Weight loss in anovulatory obese women causes resumption of ovulation and fertility in the majority of cases.[2]

4. Pesticides.  Most of the research about this is from occupational studies of farmers or factory workers who deal with high concentrations of pesticides, where they have a big effect size: 27x the risk of female infertility [3] and 3-8x increases in the risk of low sperm counts, sperm motility, and other measures of male infertility.[4]  I don’t know if you can get much risk reduction out of reducing your pesticide intake from foods; the dose is so much lower that I’m not sure it’s relevant. The bottom line may just be that if you don’t work with industrial chemicals, you have a lot to be thankful for.

5. Testicular injury.  It’s associated with a 4x increase in male infertility.[5]

6. Obesity and underweight.  Female obesity (BMI > 30) increases the risk of infertility by a factor of 3.7.[6]  Female BMI being over 25 or under 20 increases the risk of infertility by a factor of 2.2. Waist-to-hip ratio also matters; an increase of 0.1 in waist-to-hip ratio is associated with a 30% drop in the chance of conception per cycle. Hourglassy women really are more fertile, apparently.

7.  Sperm-Count-Lowering Medications.  There are many of these, including some (like androgens, anti-androgens, and some chemotherapies) that cause complete sterility until the medication is discontinued.

Smoking, drinking, and caffeine have smaller (1-2x) reductions in fertility; but in these posts, I’m just going to ignore things with small effect sizes. It’s a general epistemic hygiene thing; a lot of studies fail to replicate, there are lots of small-effect-size risks out there, and I want to concentrate on things that make the biggest difference.

So, the bottom line, in terms of things you can control that have the biggest effect sizes on fertility:

  • have safe sex, get tested for STDs, and treat them promptly
  • be nice to your testicles: avoid injuring them, and go to the doctor if they’re painful or swollen
  • avoid being overweight or underweight (for women)
  • check your medications to see if you’re taking anything that reduces sperm count or sperm function
  • don’t be a farmer.

Amusingly enough, there has actually been a study[7] showing that tight pants do not cause infertility (conducted in Italy, of course!)  Hot tubs won’t make you infertile either.[8]


[1]Thonneau, Patrick, et al. “Risk factors for female and male infertility: results of a case-control study.” Human Reproduction 7.1 (1992): 55-58

[2]Clark, A. M., et al. “Weight loss in obese infertile women results in improvement in reproductive outcome for all forms of fertility treatment.”Human Reproduction 13.6 (1998): 1502-1505.

[3]Greenlee, Anne R., Tye E. Arbuckle, and Po-Huang Chyou. “Risk factors for female infertility in an agricultural region.” Epidemiology 14.4 (2003): 429-436.

[4]Oliva, Alejandro, Alfred Spira, and Luc Multigner. “Contribution of environmental factors to the risk of male infertility.” Human Reproduction16.8 (2001): 1768-1776.

[5]Bayasgalan, G., et al. “Male infertility: risk factors in Mongolian men.” Asian J Androl 6.4 (2004): 305-11.

[6]Homan, G. F., Michael Davies, and Robert Norman. “The impact of lifestyle factors on reproductive performance in the general population and those undergoing infertility treatment: a review.” Human reproduction update 13.3 (2007): 209-223.

[7]Parazzini, F., et al. “Tight underpants and trousers and risk of dyspermia.”International journal of andrology 18.3 (1995): 137-140.

[8]KARAGAS, MARGARET R., et al. “Elevated intrascrotal temperature and the incidence of testicular cancer in noncryptorchid men.” American journal of epidemiology 129.6 (1989): 1104-1109.


2 thoughts on “How To Be Fertile

    1. So, I’ve seen some of those stats. I think it’s undeniable that heat kills sperm in the short run. Where it’s more doubtful is whether on average, in population samples, men who wear tight underwear/use hot tubs/ride bicycles/etc are less fertile over the course of a *whole year* of trying to conceive. There is about a 2x risk of infertility in some categories of men whose testes really never get a break: bakers, taxi drivers, and paraplegics. I’m betting that your average occasional briefs-wearer and sauna-goer is going to be fine.


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