Why do women over 35 have a higher incidence of multiple pregnancies?

Higher Incidence of Multiple Pregnancies in Women Over 35

Women over 35 have a higher incidence of multiple pregnancies primarily due to age-related hormonal changes that affect ovulation patterns, with a significant contribution from increased use of assisted reproductive technologies in this age group. 1

Physiological Factors Contributing to Multiple Pregnancies in Women Over 35

Hormonal Changes

• As women age, particularly over 35, follicle-stimulating hormone (FSH) levels naturally rise
• Elevated FSH can stimulate multiple follicles to develop simultaneously
• This increases the chance of releasing multiple eggs during a single ovulation cycle
• The result is a higher likelihood of dizygotic (fraternal) twins

Assisted Reproductive Technologies (ART)

• Women over 35 are more likely to use fertility treatments due to declining natural fertility 1
• These treatments significantly contribute to the rise in multiple births:
  • Ovulation-inducing medications (e.g., clomiphene citrate, gonadotropins)
  • In vitro fertilization (IVF) with multiple embryo transfers
  • Intrauterine insemination (IUI) with ovarian stimulation

Synergistic Effect of Maternal and Paternal Age

• A synergistic adverse effect exists when both partners are older 1
• Male age over 40 years has a potential negative impact on IUI success rates when the female partner is over 35 1
• This may be related to declining sperm quality with increased male age, particularly affecting:
  • Sperm motility
  • Sperm morphology
  • Semen volume

Clinical Implications of Multiple Pregnancies

Increased Risks

• Multiple gestations carry significantly higher risks compared to singleton pregnancies 1, 2:
  • Approximately 5-fold increase in fetal death
  • 7-fold increase in neonatal death
  • Higher risk of preterm delivery and complications of prematurity
  • Increased risk of growth restriction
  • Higher incidence of congenital anomalies
  • Greater risk of maternal complications (preeclampsia, gestational diabetes)

Monitoring Requirements

• Multiple gestations require more intensive monitoring than singleton pregnancies 2
• Recommended minimum monitoring includes:
  • First trimester dating scan
  • Nuchal translucency scan at 11-14 weeks
  • Detailed anatomy scan at 18-22 weeks
  • One or more third-trimester growth scans

Prevention of Multiple Pregnancies in Fertility Treatments

Ovarian Stimulation Guidelines

• When using gonadotrophins in IUI, regimens with 75 IU or lower should be used 1
• Higher doses don't improve pregnancy rates but increase multiple pregnancy rates
• Clomiphene citrate or tamoxifen are acceptable alternatives with lower multiple pregnancy rates 1

Cycle Management

• IUI should be withheld when more than two dominant follicles >15 mm or more than five follicles >10 mm are present 1
• As an alternative to cycle cancellation, aspiration of excess follicles may reduce multiple pregnancy risk 1

Conclusion

The increased incidence of multiple pregnancies in women over 35 is a multifactorial phenomenon involving both natural physiological changes and increased use of fertility treatments. Healthcare providers should counsel women in this age group about these risks and implement appropriate monitoring and management strategies to optimize outcomes.