IVF and Preterm Birth Rates
Approximately 10-11% of singleton IVF pregnancies result in preterm birth (<37 weeks), which is roughly 1.5-2 times higher than spontaneously conceived pregnancies. This represents an absolute increase of about 4-5% over natural conception rates 1, 2.
Specific Preterm Birth Rates by IVF Type
Singleton Pregnancies
- Fresh embryo transfer: 7.7% preterm birth rate 3
- Frozen embryo transfer: 6.2% preterm birth rate 3
- Overall IVF/ICSI singletons: 9.3-11.1% preterm birth rate 4, 2
- Spontaneously conceived singletons (comparison): 5.5-6.8% preterm birth rate 1, 2
The meta-analysis by Cavoretto et al. 1 provides the most robust evidence, analyzing 61,677 births and demonstrating that IVF/ICSI singleton pregnancies have a 75% increased odds of spontaneous preterm birth compared to naturally conceived pregnancies (OR 1.75,95% CI 1.50-2.03).
Multiple Gestations
The preterm birth risk is substantially higher with multiple pregnancies from IVF. The 2024 ESHRE guideline 5 emphasizes that double embryo transfer significantly increases multiple pregnancy rates, which directly correlates with preterm birth, low birthweight, and other perinatal complications. This is why elective single embryo transfer (eSET) is now the recommended standard practice.
Severity of Prematurity
Breaking down the severity of preterm births in IVF pregnancies:
- Very preterm (<34 weeks): 3.6% in IVF vs 2.2% in spontaneous conception (OR 1.78) 1
- Extremely preterm (<32 weeks): Higher incidence noted, particularly with fresh embryo transfers 3
- Late preterm (34-37 weeks): Comprises the majority of IVF preterm births 2
Risk Factors That Further Increase Preterm Birth in IVF
Several factors compound the preterm birth risk in IVF pregnancies 3:
- Endometriosis: Increases risk by 32% (aOR 1.32)
- Vanishing twin: Increases risk by 78% (aOR 1.78)
- Polycystic ovaries: Increases risk by 31% (aOR 1.31)
- Large oocyte cohort (>20 retrieved): Increases risk by 30% (aOR 1.30) in fresh transfers only
- Very advanced maternal age (≥45 years): IVF preterm birth rate of 24.4% vs 8.4% without IVF 6
Important Clinical Context
The increased preterm birth risk persists even when controlling for multiple gestations, maternal age, and parity 1, 2. This suggests that both the underlying subfertility and treatment-related factors contribute to adverse outcomes, not just the higher multiple pregnancy rates.
The sibling cohort study 2 is particularly informative because it compared IVF-conceived children to their own siblings from unassisted conceptions, controlling for genetic and environmental factors. Even in this matched analysis, IVF singletons had 1.6 times higher risk of preterm birth (aRR 1.6,95% CI 1.2-2.2), with an absolute difference of 4.3%.
Counseling Implications
When counseling patients about IVF 7:
- Inform patients that singleton IVF pregnancies carry approximately double the preterm birth risk compared to natural conception
- Emphasize that single embryo transfer dramatically reduces this risk by preventing multiple gestations 5
- Explain that fresh embryo transfers carry slightly higher preterm birth risk than frozen transfers 3
- Discuss that certain underlying conditions (endometriosis, PCOS) further elevate risk 3
The evidence quality for these findings is moderate to low 1, but the consistency across multiple large cohort studies and the biological plausibility support these recommendations for clinical practice.