Smoking in pregnancy and sons' sperm count: what the studies show

The link between smoking in pregnancy and low birth weight has been known for decades. Less well known is that several studies also tie it to how much sperm sons produce twenty years later. Below are the numbers checked against the original papers, what is solid, what gets overstated and what makes sense to do now.

Key takeaways

• Men exposed to maternal smoking before birth produce less sperm on average: about a quarter lower total sperm count in the most cited European study.
• The same study found testes roughly 1 ml smaller. A larger Danish analysis points the same way, with somewhat smaller gaps.
• Risk appears to rise with the number of cigarettes, but the dose data come from small samples with wide uncertainty.
• These are observational studies. They show a strong, repeated association, not an individual verdict. Plenty of exposed men have entirely normal results.
• The claim that effects carry over "seven generations" has no support in human data.

What the studies found

The key paper is by Tina Kold Jensen and colleagues, published in 2004 in the American Journal of Epidemiology. Between 1996 and 1999 the team collected semen samples from 1,770 young men in Denmark, Norway, Finland, Lithuania and Estonia during their military fitness exams. Mothers filled in a questionnaire with their sons, including whether they had smoked while pregnant.

After adjusting for the main confounders, including the young men's own smoking, the exposed group had on average:

Source: Jensen TK et al., Am J Epidemiol 2004 (n = 1,770). Adjusted values from the abstract.

The association showed up in each centre with one telling exception: in Lithuania only 1% of mothers had smoked in pregnancy, too few to detect anything.

The Danish follow-up

In 2011 Ravnborg and colleagues, from the same Copenhagen group, analysed 3,486 Danish men with a median age of 19, recruited between 1996 and 2006. Again, sons of mothers who smoked had fewer sperm and slightly smaller testes.

Total count was 119 million in exposed men versus 150 million in unexposed men, about 21% lower. Testis volume was 14.0 versus 14.5 ml. Exposed men also had slightly lower sperm-related hormones, a slightly earlier puberty and a slightly higher BMI.

The authors add an important caveat: these gaps may not matter clinically for any one man, but they matter for the population. Shift the average of many men downward and more of them fall below the thresholds where conceiving gets harder.

One methodological point: the 2011 study comes from the same Danish programme as the 2004 one, and the recruitment periods overlap. It is a confirmation on a larger sample, not a fully independent replication.

More cigarettes, more risk?

A third Danish study, by Jensen MS and colleagues (2005), looked for a dose-response relationship. In 522 men whose mothers' habits were known, the odds of oligozoospermia (fewer than 20 million sperm per ml) were:

With more than 10 cigarettes a day the estimated odds were 2.6 times those of sons of non-smokers. The direction is clear, but the limits matter. The interval is wide, from 1.2 to 5.8. The 1 to 10 cigarettes group did not reach statistical significance, and neither did the differences in mean concentration.

Another Danish cohort, which followed 347 sons of women interviewed during pregnancy, found an inverse relationship with total sperm count. Above 19 cigarettes a day, total count was 38% lower and semen volume 19% lower. Only the overall trend and the volume difference were statistically significant, though.

Why the effect might last

The testis forms during fetal life. That is when Sertoli cells, which later support sperm production, multiply, along with the precursors of germ cells. A 2012 review concludes that smaller testes and poorer semen in exposed men point to a toxic effect on exactly these cells.

Mouse experiments point the same way. When mothers were exposed to smoke through pregnancy and nursing, their adult sons had fewer sperm stem cells, more DNA damage and lower fertility. It is an animal model: useful for the mechanism, not directly transferable to humans.

What remains open is that no human study has tested whether, or how much, these differences can recover. It is fair to say the harm is plausibly tied to an early window of development. It is not proven to be irreversible in every case.

Three generations, not seven

One biological point that circulates is correct. During pregnancy the fetus already forms its germ cells. So a pregnancy involves the mother, the fetus and, in a sense, the fetus's future children.

Going from there to "seven generations" is a huge leap. The studies above measure one generation, the sons. There are no human data showing effects on grandchildren, let alone beyond.

What the research actually says

This article was prompted by a post from Marco Guercioni (@the___doc). Here are its main claims, checked against the abstracts.

• Confirmed — "Sons of smoking mothers have about a quarter fewer sperm." True for total sperm count (−24.5%) in the 2004 European study.
• Confirmed — "1,770 young men in 5 countries, concentration −20.1%, testes 1.15 ml smaller." The figures match the abstract.
• Needs context — "Replicated in 3,486 Danes in 2011." The differences point the same way but are somewhat smaller (−21% total count, −0.5 ml testis volume), and both samples come from the same Danish programme.
• Needs context — "Above 10 cigarettes a day the risk of oligozoospermia doubles." The estimate is actually 2.6, but it comes from 522 men, with an interval of 1.2 to 5.8 and a non-significant middle group.
• Needs context — "It can't be recovered because these cells are set in fetal life." The mechanism is plausible and backed by animal models, but irreversibility has not been measured in humans.
• Confirmed — "A pregnancy involves three generations." This is basic embryology: the fetus forms its germ cells during gestation.
• Not supported — "Seven generations." There is no human evidence beyond the first generation of sons.

The post also says one figure is often repeated incorrectly. We don't know which one it meant. The most common mix-up we see is swapping total count (−24.5%) with concentration (−20.1%). They are different measures, and the "quarter fewer" applies only to the first.

What you can do

This is not about blaming anyone. Quitting is hard, nicotine dependence is a medical condition, and many mothers in these studies were pregnant in the 1970s and 1980s, when the information was different. The data are here to guide choices made today.

1. If you are trying to conceive or are pregnant, talk to your doctor or midwife. Quitting before or early in pregnancy is best, but cutting down is still better than doing nothing.
2. Get support rather than going it alone. Most countries run free quitlines and stop-smoking services; your doctor or pharmacist can point you to options that are suitable in pregnancy.
3. Involve your partner. Second-hand smoke at home matters, and a father's own smoking affects his semen quality too.
4. If you were exposed before birth, there is no reason to panic. If you and your partner have been trying for 12 months without success, a urologist or andrologist can order a semen analysis, the only way to know your actual numbers.

FAQ

My mother smoked while pregnant. Will I definitely be less fertile?

No. The data describe averages across thousands of men. Many exposed men have normal results and become fathers without trouble. If you are worried or have been trying for a while, a semen analysis is the test that actually answers the question.

Which matters more, the mother's smoking or the son's own smoking as an adult?

In the studies above, the effect of prenatal exposure remained after accounting for the young men's own smoking. Adult smoking has its own effects on semen quality, so quitting is worthwhile either way.

Are e-cigarettes a safe alternative in pregnancy?

The studies discussed here are about conventional cigarettes. Data on e-cigarettes in pregnancy and sons' fertility are still scarce; it is a topic to raise with your obstetrician.

Do the effects only concern sons?

This article is about semen quality, so it covers sons. Smoking in pregnancy has well-documented effects on other aspects of a child's health regardless of sex.

Sources

1. Jensen TK et al., 2004. Association of in utero exposure to maternal smoking with reduced semen quality and testis size in adulthood: a cross-sectional study of 1,770 young men from the general population in five European countries. Am J Epidemiol 159(1):49–58. PMID 14693659 · DOI 10.1093/aje/kwh002
2. Ravnborg TL et al., 2011. Prenatal and adult exposures to smoking are associated with adverse effects on reproductive hormones, semen quality, final height and body mass index. Hum Reprod 26(5):1000–11. PMID 21335416 · DOI 10.1093/humrep/der011
3. Jensen MS et al., 2005. Lower sperm counts following prenatal tobacco exposure. Hum Reprod 20(9):2559–66. PMID 15919771 · DOI 10.1093/humrep/dei110
4. Ramlau-Hansen CH et al., 2007. Is prenatal exposure to tobacco smoking a cause of poor semen quality? A follow-up study. Am J Epidemiol 165(12):1372–9. PMID 17369608 · DOI 10.1093/aje/kwm032
5. Virtanen HE, Sadov S, Toppari J, 2012. Prenatal exposure to smoking and male reproductive health. Curr Opin Endocrinol Diabetes Obes 19(3):228–32. PMID 22499223 · DOI 10.1097/MED.0b013e3283537cb8
6. Sobinoff AP et al., 2014. Damaging legacy: maternal cigarette smoking has long-term consequences for male offspring fertility. Hum Reprod 29(12):2719–35. PMID 25269568 · DOI 10.1093/humrep/deu235 (mouse study)

Article inspired by a post by Marco Guercioni (@the___doc). Sources retrieved via PubMed.

Note: for information only; not a substitute for advice from a doctor or qualified professional.