In full-term elective caesarian sections, fetal flow of adrenal substrate 
steroids to products differs by sex, with males (M) in molar equilibrium whereas 
females (F) add net molarity and synthesize more cortisol. Using the same 
sampling design, paired, full-term, arterial, and venous umbilical cord samples 
and intrapartum chart records were obtained at the time of vaginal delivery (N = 
167, 85 male) or emergency C-section (N = 38, 22 male). Eight steroids were 
quantified by liquid chromatography coupled to tandem mass spectrometry (adrenal 
glucocorticoids [cortisol, corticosterone], sequential cortisol precursor 
steroids [17-hydroxyprogesterone, 11-deoxycortisol], cortisol and corticosterone 
metabolites [cortisone and 11-dehydrocorticosterone], and gonadal steroids 
[androstenedione, testosterone]). Fetal sex was not significant in any analytic 
models. Going through both phase 1 and phase 2 labor increased fetal adrenal 
steroidogenesis and decreased male testosterone relative to emergency C-sections 
that do not reach stage 2 of labor (ie, head compressions) and elective 
C-sections with no labor. Sum adrenal steroid molarity arriving in venous serum 
was almost double the equivalent metric for deliveries without labor. No effects 
of operative vaginal delivery were noted. Maternal regional anesthetic 
suppressed venous concentrations, and fetal synthesis replaced that steroid. 
Approximate molar equivalence between substrate pool depletion and net 
glucocorticoid synthesis was seen. Paired venous and arterial umbilical cord 
serum has the potential to identify sex differences that underlie antenatal 
programming of hypothalamic-pituitary-adrenal axis function in later life. 
However, stage 2 labor before the collection of serum, and regional anesthetic 
for the mother, mask those sex differences.