Effects of Dydrogesterone on the Fetus
Based on current evidence, dydrogesterone appears to have a reassuring fetal safety profile with no confirmed increased risk of major birth defects, though recent pharmacovigilance data warrant cautious monitoring for potential signals of hypospadias and congenital heart defects.
Safety Profile Overview
The most comprehensive meta-analysis of randomized controlled trials found no causal association between first-trimester dydrogesterone use and fetal abnormalities (RR 0.96; 95% CI 0.57-1.62) 1. This analysis included six RCTs published since 2005 and directly contradicts two low-quality retrospective studies (one of which was retracted) that had suggested a link.
Key Fetal Outcomes from Clinical Trials
No maternal complications or congenital anomalies could be linked to dydrogesterone usage during assisted reproductive techniques (ART) or during early pregnancy to prevent recurrent miscarriages 2. The large phase III trial comparing dydrogesterone to vaginal progesterone for luteal phase support showed no new fetal safety concerns 3.
Emerging Safety Signal - Important Caveat
A 2025 pharmacovigilance analysis of the WHO global safety database (VigiBase) identified a disproportionate reporting of birth defects with dydrogesterone exposure, particularly:
- Hypospadias (genital defects)
- Congenital heart defects
The reporting odds ratio was significantly elevated when comparing dydrogesterone to:
- Any other drug (ROR 5.4,95% CI 3.9-7.5)
- Any other ART drug (ROR 6.0,95% CI 4.2-8.5)
- Progesterone specifically (ROR 5.4,95% CI 3.7-7.9) 4
Critical limitation: This represents a pharmacovigilance signal, not proof of causation. Under-reporting and confounding factors inherent to spontaneous reporting systems limit interpretation. However, the consistency across sensitivity analyses suggests this warrants further investigation.
Animal Studies
Mouse studies using doses comparable to human ART protocols showed:
- No interference with placental development, fetal growth, or fetal survival
- Normal offspring growth and fertility
- One concerning finding: Altered spermiogram in male progeny
- Shifted sex ratio favoring female offspring 5
While these findings may not directly translate to humans, they suggest caution is warranted.
Clinical Context
When Dydrogesterone is Used:
- Luteal phase support in ART cycles
- Prevention of recurrent miscarriage
- Threatened miscarriage management
- Preterm birth prevention (though evidence for efficacy is insufficient 6)
Comparison to Standard Care:
The evidence base for dydrogesterone is primarily from preterm birth prevention studies using other progestogens (17P, vaginal progesterone), which showed no effect on perinatal morbidity and mortality in multiple populations [@1-7@]. Dydrogesterone has been positioned as non-inferior to vaginal progesterone for pregnancy outcomes at 12 weeks gestation 3.
Practical Recommendations
For current use in ART/early pregnancy support:
- Dydrogesterone can be used when indicated based on current RCT evidence showing no increased risk
- Inform patients about the recent pharmacovigilance signal while emphasizing its preliminary nature
- Consider preferential use of vaginal progesterone when equally appropriate, given its longer safety track record
For monitoring:
- Detailed fetal anatomic ultrasound with specific attention to cardiac structures and genitalia
- Document dydrogesterone exposure timing and duration
- Report any suspected birth defects to pharmacovigilance systems
Avoid routine use for preterm birth prevention, as efficacy remains unproven and the risk-benefit ratio is uncertain 6.
Evidence Quality Assessment
The discrepancy between RCT data (showing no risk) and pharmacovigilance data (showing increased reporting) represents a genuine uncertainty. RCTs provide higher-quality evidence for causation but may lack power to detect rare events. The 2025 pharmacovigilance analysis 4 is the most recent evidence and cannot be dismissed despite methodological limitations.
The prudent approach: Continue using dydrogesterone when clinically indicated (threatened miscarriage, ART luteal support) while maintaining heightened surveillance for the specific defects identified in the safety signal, pending definitive prospective studies.