Invited commentary: the hidden population in perinatal epidemiology

Area of Science:

• Perinatal epidemiology
• Obstetric research
• Fetal development studies

Background:

• The concept of 'fetuses at risk' (fetuses in utero) has been proposed to enhance understanding of perinatal pathology.
• Historically, it was suggested as the optimal denominator for intrapartum stillbirths.
• Recent work explores fetal growth in utero compared to premature infants, addressing biases from impaired fetal growth selection.

Discussion:

• While the 'fetuses at risk' concept is valuable, its application as a denominator for postnatal outcomes like neonatal mortality or cerebral palsy is problematic.
• The denominator population (fetuses in utero) differs significantly from the outcome population (postnatally adapted infants), especially premature infants.
• Growth standards derived from ultrasonography are often based on limited, selected samples, potentially introducing bias.

Key Insights:

• The 'fetuses at risk' perspective offers a novel approach to perinatal process analysis.
• Failure to account for selection bias in fetal growth can skew research findings.
• The utility of 'fetuses at risk' as a denominator for postnatal outcomes is limited by the lack of postnatal adaptation in the denominator group.

Outlook:

• Further refinement of the 'fetuses at risk' concept is needed for accurate perinatal research.
• Careful consideration of the denominator population is crucial when analyzing perinatal and postnatal outcomes.
• This concept has significant implications for clinical practice and future obstetric research, demanding cautious application.