# Diurnal Effects on the Fraction of Fetal Cell‐Free DNA in Maternal Plasma

## TL;DR

This study shows that the amount of fetal DNA in a pregnant woman's blood varies depending on the time of day, which could help improve non-invasive prenatal testing.

## Contribution

The study reveals a novel diurnal variation in fetal cell-free DNA fraction, independent of known factors like BMI and gestational age.

## Key findings

- Fetal fraction increases by about 10% in the afternoon compared to the morning.
- Blood draw timing is a stronger predictor of fetal fraction than gestational age or fetal sex.
- The effect of time of day on fetal fraction is statistically significant and clinically relevant.

## Abstract

The discovery of fetal cell‐free DNA (cfDNA) has revolutionized prenatal diagnostics through non‐invasive prenatal testing (NIPT), which depends on accurately measuring the fetal fraction (FF) in maternal plasma. While FF is known to be influenced by maternal and fetal factors, the impact of intraday rhythms remains unclear. This study investigated whether FF varies based on blood draw timing.

Data from 2519 euploid singleton pregnancies undergoing NIPT were analyzed. Key variables included maternal age, BMI, gestational age, fetal sex, and blood draw timing (06:50–21:00). FF was measured using the Harmony Prenatal IVD Test. Multiple linear regression identified independent predictors of FF, while intraday variation was assessed using Mann‐Whitney U tests and boxplots.

FF showed a significant positive relationship with blood draw timing (β = 0.00176 per hour, p < 0.005), with afternoon values approximately 10% higher than morning values (∼0.01 difference). Other predictors included BMI (negative), gestational age (positive), and fetal sex (higher in females). Blood draw timing appeared to be a stronger predictor of FF than gestational age or fetal sex, second only to BMI.

This novel finding demonstrates diurnal variation in FF, suggesting that optimizing blood draw timing could improve NIPT accuracy, particularly in borderline cases. Further research is needed to confirm the clinical implications.