PurposeThe purpose of this study was to determine the 4 -hydroxycholesterol to cholesterol ratio in mothers and neonates at the time of birth and four months post partum.Method 21 mothers and 22 neonates were recruited at the delivery ward at Karolinska University Hospital, Huddinge,Sweden. Blood samples taken from mothers and neonates at birth and four months post partum were analyzed for 4 -hydroxycholesterol and cholesterol. ResultsThe median plasma concentration of 4 -hydroxycholesterol was higher in mothers at delivery (50 ng/mL) compared to healthy non-pregnant women (29 ng/mL). The pregnant women had a higher median cholesterol concentration (6.2 mmol/L) compared to healthy non-pregnant women (4.6 mmol/L) but this could only partly explain the increased 4 -hydroxycholesterol. The major cause is an increased CYP3A activity during pregnancy. The median 4 -hydroxycholesterol/cholesterol ratio•10 4 was elevated in mothers at time of birth compared to non-pregnant women (0.19 and 0.15, respectively) but decreased to 0.15 four months post partum.Neonates had a median 4 -hydroxycholesterol/cholesterol ratio•10 4 (0.19) comparable to adults already at birth, but lower 4 -hydroxycholesterol (12 ng/mL) and cholesterol (1.8 mmol/L) concentrations. ConclusionPregnancy leads to increased CYP3A enzyme activity as determined by the 4 -hydroxycholesterol/cholesterol ratio. Neonates have low 4 -hydroxycholesterol and cholesterol concentrations but similar total CYP3A activity as adults already at birth.