Wilson's Rules & Teratogen Principles

A teratogen is any agent (drug, infection, maternal metabolic disturbance, or physical exposure) that disrupts normal embryologic or fetal development. Wilson's rules, articulated in the 1960s and revised since, give a six-principle framework that explains why the same agent can produce different phenotypes (or no phenotype at all) depending on dose, timing, and host susceptibility. These principles also predict which exposures matter clinically and which counsel as reassuring.

1. Susceptibility depends on the conceptus's genotype and on maternal metabolism. The same drug at the same dose produces different effects in different fetuses: alcohol dehydrogenase polymorphisms modulate FASD risk; CYP enzyme variants affect phenytoin metabolism and therefore phenytoin embryopathy risk.
2. Susceptibility varies with developmental stage at the time of exposure. The all-or-none period (weeks 0–2) is binary: loss or unaffected. Organogenesis (weeks 3–8) produces structural malformations. The fetal period (weeks 9–term) typically produces growth restriction, brain/kidney functional deficits, and disrupted histogenesis rather than gross structural anomalies.
3. Teratogens act by specific mechanisms. Folate antagonism (methotrexate, valproate, phenytoin), vascular disruption (cocaine, amniotic bands), receptor binding (retinoids, ACE inhibitors), DNA damage (radiation), and methylation interference (alcohol, valproate) each produce recognizable patterns.
4. The dose-response relationship matters. Most teratogens have a threshold dose below which no detectable effect is seen. Above threshold, severity (and frequency) increase with dose. Alcohol is the major exception: no threshold has been established; even low-dose chronic exposure can produce neurodevelopmental effects.
5. Manifestations span a spectrum: embryonic death, structural malformation, growth restriction, and functional/behavioral deficit. The same agent at different doses or different timings can produce any of these.
6. Access to the agent matters. Lipophilic, low-molecular-weight, and protein-unbound drugs cross the placenta freely. Most clinically used drugs do cross. Exceptions include insulin (large protein), heparin / LMWH (highly polar), and most monoclonal antibodies in the first trimester (transfer increases as gestation advances and the IgG-transport machinery comes online).

The three-window framework is the single most important concept in the chapter; every counseling decision flows from it:

• All-or-none (weeks 0–2 post-conception, ~2–4 weeks of gestational age). Exposure causes embryonic death or has no effect; there's no in-between because the embryo is still totipotent and replaces damaged cells. Reassuring counseling for very-early exposures before missed period.
• Organogenesis (weeks 3–8 post-conception, ~5–10 weeks gestational age). Each organ has its own narrower window:
  • Heart: weeks 3–6
  • Neural tube: weeks 3–4 (anterior neuropore day 25; posterior day 27)
  • Limb buds: weeks 4–7
  • Palate: weeks 7–12
  • External genitalia: weeks 7–12 (and beyond for masculinization)
• Fetal period (weeks 9–term). Structural anomalies are uncommon; growth restriction, microcephaly, brain functional deficits, and disrupted histogenesis predominate. Late-trimester ACE-inhibitor or NSAID exposure causes characteristic injury patterns (renal hypoplasia, oligohydramnios; ductal closure) rather than malformation.

When a patient presents with a known exposure, the questions to answer (in order):

1. What was the agent and dose? Brand-name drug, typical dose; for environmental/food exposures, frequency.
2. What was the gestational timing? Convert "weeks pregnant" to post-conception weeks (subtract 2). Match to the relevant critical window for each organ.
3. What's the established teratogenic risk? Look up TERIS, MotherToBaby, ReproTox. Distinguish evidence of risk from absence of safety data.
4. What's the background risk? Always anchor counseling against the ~3% baseline congenital-anomaly rate. A 2× increase from ~3% to 6% sounds different than "doubled risk."
5. What surveillance helps? Targeted anatomy ultrasound at 18–22 weeks, fetal echocardiography for cardiac teratogens, AFP/MSAFP if neural tube risk.

• Paternal exposures: limited evidence for teratogenesis, though increasing data on paternal age effects (achondroplasia and other FGFR3 mutations rise with paternal age; not a teratogenic mechanism but a similar counseling problem).
• Lactation: a separate question from teratogenesis. Most drugs reach breast milk at <10% maternal serum levels; safety during lactation must be assessed separately.
• "Behavioral teratogens": agents that alter neurodevelopment without producing gross malformation (alcohol, methylmercury, lead, polychlorinated biphenyls). These often affect the fetal period rather than organogenesis.