• Home
  • Pamphlets and Brochures
  • Donate now
  • Frequently Asked Questions
  • Please read

Our Helplines

Motherisk Helpline
1-877-439-2744 (Toll-free)
416-813-6780 (Toronto and GTA)

Alcohol and Substance

Bias against the null hypothesis - Scaring pregnant women about drugs in pregnancy

Gideon Koren, MD FRCPC FACMT, Svetlana Madjunkova, MD PhD and Caroline Maltepe

May 2014



Question Since the thalidomide disaster, medicine is practised as if every drug is teratogenic, when in fact very few medications are. Pregnant women are often ready to refuse treatment even for life-threatening conditions owing to misinformation and misperceptions about fetal risks. How can I reassure my patients and prevent misinformation from affecting their treatment?


Answer Physicians must provide evidence-based counseling to their patients. For example, antihistamines for morning sickness have been proven safe in numerous studies, but are commonly the subject of media reports overstating the risks to the fetus. Family physicians and obstetricians must take an active role in preventing pregnant patients from being misinformed.


Depuis le désastre de la thalidomide, on pratique la médecine comme si tous les médicaments étaient tératogènes, alors qu'en réalité, très peu le sont. Les femmes enceintes sont souvent prâtes à refuser un traitement même en cas de problèmes qui menacent leur vie à cause de renseignements et de perceptions erronés au sujet des risques pour le fœtus. Comment puis-je rassurer mes patientes et empêcher que la désinformation nuise à leur traitement?


Les médecins doivent fournir à leurs patientes des conseils fondés sur des données probantes. Par exemple, les antihistaminiques pour les nausées matinales se sont révélés sécuritaires dans nombreuses études, mais ils font souvent l'objet de reportages médiatiques exagérant leurs risques pour le fœtus. Les médecins de famille et les obstétriciens doivent jouer un rôle actif pour empêcher que les patientes enceintes soient mal informées.

Histamine-type 1 blockers (antihistamines) have been widely used in different products aimed at treating morning sickness, which affects up to 80% of all pregnancies. 1 The fetal safety of antihistamines has been repeatedly documented in numerous studies, and 5 different meta-analyses have corroborated the safety of their use. 2-6 In fact, one of these analyses 4 even pointed out that antihistamines had an apparent protective effect against malformations. The authors of 2 studies opined that it is not the antihistamines that protect the baby, but rather the nausea and vomiting of pregnancy itself, which is known to confer favourable pregnancy outcomes including prevention of miscarriages and malformations, and to have a beneficial effect on long-term development. 7,8 A recent study reanalyzed this original meta-analysis, and excluded 2 of 24 studies, which accounted for more than 40 000 women, because the authors could not locate the articles (the original paper had several references out of order and one missing). The authors concluded that antihistamines do not confer a protective effect, but are still safe for the fetus. 9 An analysis that includes the missing references from the original meta-analysis corroborates the initial results, showing an apparent protective effect of antihistamines. 10

The authors of the new, erroneous re-analysis brought it to the attention of a national newspaper, which published a headline insinuating that antihistamines were not safe in pregnancy, 11 prompting readers to react in panic. At the time the newspaper's report was published, the journalist was aware of the errors in the re-analysis, but he did not include these details in his report. Moreover, he did not make it clear that "lack of protective effect" of antihistamines is a long way from "risk," and that 4 other meta-analyses confirmed the safety of antihistamines.

Bias in scientific information

Because the Motherisk program is consulted by up to 200 women and their health professionals every day about the use of drugs and exposure to chemicals in pregnancy, we are painfully aware of the misinformation and misperceptions that pregnant women and their families encounter. Bias against the null hypothesis is the term used to describe the tendency to report adverse events of drugs more often than reporting on their safety. We have shown that a study of a medication that shows no increase in risk is much less likely to get published in meeting abstracts and journals, and to be reported by the media. 12-16 As a result, the medical literature is often distorted toward alarming rather than relieving fears, even with use of safe drugs such as antihistamines for morning sickness. Box 1 presents the numerous ways in which this distortion is created, promoted, and sustained. 12-16

Not surprisingly, pregnant women exposed to nonteratogenic drugs tend to assume these medications carry high fetal risks, and this misperception leads many of them to consider terminating otherwise-wanted pregnancies. 17,18

We have shown that more vulnerable women (eg, women with depression, single mothers) are more likely to be negatively affected by misinformation, with higher tendencies to terminate otherwise-wanted pregnancies. 19,20 The silver lining here is that evidence-based counseling of pregnant women can avoid terminations. 18

Box 1.

How the bias against the null hypothesis is created

The following situations explain how the bias against the null hypothesis is created:

  • Abstracts are more likely to be presented at meetings if they have adverse results than if they show no increase in risk
  • Papers are more likely to be published if they report on adverse results than if they report on results that show no increase in risk
  • Media reports are much more likely for papers that find adverse results than for those that show no increase in risk
  • Physicians are much more likely to cite adverse results than results that show no increase in risk in subsequent research
  • Journal reviewers are more likely to accept articles with adverse results for publication than articles with results that show no increase in risk Data from Koren et al. 12-16


Physicians in general, and obstetricians in particular, must take an active role in preventing misinformation from adversely affecting the management of pregnant patients. The Motherisk program is always pleased to talk to and counsel your patients directly in cases in which physicians believe this can help women and their families.


Motherisk questions are prepared by the Motherisk Team at The Hospital for Sick Children in Toronto, Ont. Dr. Koren is Director and Dr. Madjunkova and Ms. Maltepe are members of the Motherisk Program. Dr Koren is supported by the Research Leadership for Better Pharmacotherapy during Pregnancy and Lactation. He holds the Ivey Chair in Molecular Toxicology in the Department of Medicine at the University of Western Ontario in London.

Do you have questions about the effects of drugs, chemicals, radiation, or infections in women who are pregnant or breastfeeding? We invite you to submit them to the Motherisk Program by fax at 416 813-7562; they will be addressed in future Motherisk Updates.

Published Motherisk Updates are available on the Canadian Family Physician website (www.cfp.ca) and also on the Motherisk website.

View abstract »»

Competing interests
Motherisk research is supported by Duchesnay Inc, manufacturers of Diclectin (pyridoxine-doxylamine) for morning sickness.

Copyright © the College of Family Physicians of Canada
Can Fam Physician
Vol. 60, No. 5, May 2014 441-442
Copyright © 2014 by The College of Family Physicians of Canada


  1. Maltepe C, Koren G. The management of nausea and vomiting of pregnancy and hyperemesis gravidarum—a 2013 update. J Popul Ther Clin Pharmacol 2013;20(2):e184-92. Epub 2013 Jul 14. Medline
  2. Einarson TR, Leeder JS, Koren G. A method for meta-analysis of epidemiological studies. Drug Intell Clin Pharm 1988;22(10):813-24. Search Google Scholar
  3. McKeigue PM, Lamm SH, Linn S, Kutcher JS. Bendectin and birth defects: I. A meta-analysis of the epidemiologic studies. Teratology 1994;50(1):27-37. CrossRef | Medline | Search Google Scholar
  4. Seto A, Einarson T, Koren G. Pregnancy outcome following first trimester exposure to antihistamines: meta-analysis. Am J Perinatol 1997;14(3):119-24. Medline | Search Google Scholar
  5. Magee LA, Mazzotta P, Koren G. Evidence-based view of safety and effectiveness of pharmacologic therapy for nausea and vomiting of pregnancy (NVP). Am J Obstet Gynecol 2002;186(5 Suppl):S256-61. CrossRef | Medline | Search Google Scholar
  6. Kutcher JS, Engle A, Firth J, Lamm SH. Bendectin and birth defects: II. Ecological analyses. Birth Defects Res A Clin Mol Teratol 2003;67(2):88-97. CrossRef | Medline | Search Google Scholar
  7. Nulman I, Rovet J, Barrera M, Knittel-Keren D, Feldman BM, Koren G. Long-term neurodevelopment of children exposed to maternal nausea and vomiting of pregnancy and diclectin. J Pediatr 2009;155(1):45-50. e2. Epub 2009 Apr 24. Medline | Search Google Scholar
  8. Brandes JM. First-trimester nausea and vomiting as related to outcome of pregnancy. Obstet Gynecol 1967;30(3):427-31. Medline | Search Google Scholar
  9. Chin JW, Gregor S, Persaud N. Re-analysis of safety data supporting doxylamine use for nausea and vomiting of pregnancy. Am J Perinatol 2013 Dec 9. Epub ahead of print. Search Google Scholar
  10. Seto A. Meta-analysis of adverse neonatal effects due to maternal exposure to antihistamines [master’s thesis]. Toronto, ON: University of Toronto; 1992. Available from: Motherisk Accessed 2014 Mar 25.
  11. Blackwell T. Diclectin, popular morning sickness drug, less safe than key study said, new report warns. National Post 2013 Dec 19. Available from: National Post
  12. Koren G, Fernandes A. Reviewers’ bias against the null hypothesis: the reproductive hazard of binge drinking. J Popul Ther Clin Pharmacol 2010;17(2):e281-3. Epub 2010 Jul 20. Medline | Search Google Scholar
  13. Koren G, Nickel S. Sources of bias in signals of pharmaceutical safety in pregnancy. Clin Invest Med 2010;33(6):E349-55. Medline | Search Google Scholar
  14. Koren G. Bias against the null hypothesis in maternal-fetal pharmacology and toxicology. Clin Pharmacol Ther 1997;62(1):1-5. CrossRef | Medline | Search Google Scholar
  15. Koren G, Graham K, Shear H, Einarson T. Bias against the null hypothesis: the reproductive hazards of cocaine. Lancet 1989;2(8677):1440-2. CrossRef | Medline | Search Google Scholar
  16. Koren G, Klein N. Bias against negative studies in newspaper reports of medical research. JAMA 1991;266(13):1824-6. CrossRef | Medline | Search Google Scholar
  17. Koren G, Bologa M, Long D, Feldman Y, Shear NH. Perception of teratogenic risk by pregnant women exposed to drugs and chemicals during the first trimester. Am J Obstet Gynecol 1989;160(5 Pt 1):1190-4. Medline | Search Google Scholar
  18. Koren G, Pastuszak A. Prevention of unnecessary pregnancy terminations by counselling women on drug, chemical, and radiation exposure during the first trimester. Teratology 1990;41(6):657-61. CrossRef | Medline | Search Google Scholar
  19. Walfisch A, Sermer C, Matok I, Einarson A, Koren G. Perception of teratogenic risk and the rated likelihood of pregnancy termination: association with maternal depression. Can J Psychiatry 2011;56(12):761-7. Medline | Search Google Scholar
  20. Bonari L, Koren G, Einarson TR, Jasper JD, Taddio A, Einarson A. Use of anti-depressants by pregnant women: evaluation of perception of risk, efficacy of evidence based counseling and determinants of decision making. Arch Womens Ment Health 2005;8(4):214-20. Epub 2005 Jun 17. CrossRef | Medline | Search Google Scholar
Valid XHTML 1.0 Transitional [Valid RSS]

Motherisk is a proud affiliate of MotherToBaby, an information service of the non-profit Organization of Teratology Information Specialists (OTIS) across North America.

* - "MOTHERISK - Treating the mother - Protecting the unborn" is an official mark of The Hospital for Sick Children. All rights reserved.

The information on this website is not intended as a substitute for the advice and care of your doctor or other health-care provider. Always consult your doctor if you have any questions about exposures during pregnancy and before you take any medications.

Copyright © 1999-2018 The Hospital for Sick Children (SickKids). All rights reserved.

The Hospital for Sick Children (SickKids) is a health-care, teaching and research centre dedicated exclusively to children; affiliated with the University of Toronto. For general inquires please call: 416-813-1500.

  |  Contact SickKids  |  Terms of Use