| Factor | Category (Risk/Protective) | Effect Size (OR if available) | Mechanism | Gene-Environment Interaction | Source |
|---|---|---:|---|---|---|
| Maternal smoking | Risk | OR 1.3-1.8 overall; up to OR 4.2 for bilateral cases with >25 cigarettes/day | Nicotine-mediated vasoconstriction may impair uteroplacental blood flow and fetal oxygen delivery; smoke exposure is also linked to DNA methylation changes and oxidative-stress/cell-cycle dysregulation during craniofacial development | Reported interactions with **MSX1**, **RUNX2**, **TGFA**, **TGFB3**, and **BMP4** susceptibility variants | (pqac-00000017, pqac-00000020, pqac-00000022, pqac-00000023) |
| Folate deficiency / low periconceptional folate | Risk | OR 4.36 | Reduced one-carbon metabolism and methyl-donor availability can impair neural crest/palatal development and alter DNA methylation during embryogenesis | Strongly linked to **MTHFR** and other folate-pathway variants such as **SHMT1** | (pqac-00000017, pqac-00000018, pqac-00000024) |
| Maternal alcohol exposure | Risk | Not consistently quantified in retrieved evidence | Alcohol can inhibit retinoic acid biosynthesis via acetaldehyde and disrupt craniofacial morphogenesis; first-trimester binge drinking is particularly implicated | Gene-environment interaction frameworks reported; **PDGFRA** noted as protective in animal models against alcohol-related craniofacial defects | (pqac-00000018, pqac-00000021, pqac-00000023) |
| Maternal obesity (including grade II/extreme obesity) | Risk | OR 1.32 | Likely contributes through metabolic/inflammatory dysregulation during early development | No specific interaction quantified in retrieved evidence | (pqac-00000017, pqac-00000021) |
| Pre-pregnancy diabetes mellitus | Risk | OR 1.96 | Hyperglycemia and metabolic teratogenicity may perturb craniofacial development | No specific interaction quantified in retrieved evidence | (pqac-00000017) |
| Pre-pregnancy hypertension | Risk | OR 1.17 | Vascular and placental dysfunction may adversely affect embryonic craniofacial development | No specific interaction quantified in retrieved evidence | (pqac-00000017) |
| Assisted reproductive technology | Risk | OR 1.40 | Mechanism uncertain; may reflect parental/subfertility factors or early embryologic influences | No specific interaction quantified in retrieved evidence | (pqac-00000017) |
| TCDD / dioxins | Risk | Not quantified in retrieved evidence | TCDD can inhibit palatal fusion and acts through epigenetic mechanisms including histone acetylation and microRNA dysregulation | Interacts with genetically susceptible developmental pathways; exact human genotype modifier not specified in retrieved evidence | (pqac-00000018) |
| Retinoic acid excess / vitamin A toxicity | Risk | Not quantified in retrieved evidence | Excess retinoids impair key signaling pathways during palatogenesis and are recognized teratogens; dysregulated RA signaling disrupts facial process fusion | Alcohol-related RA deficiency and retinoid pathway susceptibility may modify risk | (pqac-00000018, pqac-00000022) |
| Air pollutants (PM2.5, PM10, ozone, carbon monoxide) | Risk | Not quantified in retrieved evidence | Early gestational pollutant exposure may induce oxidative stress and developmental disruption | No specific interaction quantified in retrieved evidence | (pqac-00000017) |
| Maternal stress | Risk | Not quantified in retrieved evidence | Psychophysiologic stress is associated with increased risk in observational studies; likely mediated through neuroendocrine and inflammatory pathways | No specific interaction quantified in retrieved evidence | (pqac-00000021, pqac-00000022) |
| Folic acid supplementation (>=400 µg/day during first 4 weeks/periconception) | Protective | Protective; inverse association reported | Supports folate-dependent one-carbon metabolism, nucleotide synthesis, and methylation during neural crest/palatal development | Particularly relevant in carriers of folate-pathway risk variants such as **MTHFR** | (pqac-00000017, pqac-00000018, pqac-00000019, pqac-00000024) |
| MTHFR polymorphisms (e.g., C677T, 1298A>C) | Genetic susceptibility / risk modifier | Not uniform across studies | Reduced folate utilization and lower SAM availability may lead to hypomethylation and increased susceptibility to clefting | Modifies impact of folate deficiency and may alter response to folic acid supplementation | (pqac-00000017, pqac-00000018, pqac-00000024) |
| MSX1 genotype with maternal smoking | Gene-environment risk interaction | 7.16-fold increased risk | Smoking exposure plus cleft-susceptibility genotype likely amplifies developmental signaling disruption during lip/palate fusion | **MSX1 × smoking** | (pqac-00000022) |
| RUNX2 variant with maternal smoking | Gene-environment risk interaction | Increased risk reported; exact OR not provided in retrieved evidence | Smoking appears to enhance the effect of craniofacial regulatory variation on cleft risk | **RUNX2 × smoking** | (pqac-00000017, pqac-00000022) |
| TGFA / TGFB3 / BMP4 polymorphisms with smoking | Gene-environment risk interaction | Increased risk reported; exact ORs not provided in retrieved evidence | Tobacco exposure may interact with growth-factor signaling and methylation-sensitive pathways central to fusion and epithelial-mesenchymal signaling | **TGFA × smoking**, **TGFB3 × smoking**, **BMP4 × smoking** | (pqac-00000020, pqac-00000022) |


*Table: This table summarizes major environmental and gene-environment contributors to cleft lip/palate risk, including effect sizes where available. It is useful for etiologic annotation, prevention planning, and linking modifiable exposures to molecular susceptibility pathways.*