OBJECTIVE Little is known of the hormone environment of the developing early human embryo. We have therefore measured selected steroids in the intrauterine fluids of early pregnancy.DESIGN Measurement of progesterone, 17α‐hydroxyprogesterone, oestradiol‐17β, testoste rone, androstenedione, cortisol and dehydroepiandrosterone sulphate in matched samples of coelomic fluid, amniotic fluid and maternal serum collected before pregnancy termination from 12 women between 8 and 12 weeks gestation.RESULTS Mean concentrations of progesterone, oestradiol‐17β and 17α‐hydroxyprogesterone in coelomic fluid were respectively 20, 6 and 2 times greater than in maternal serum and 8, 13 and 2.6 times those in amniotic fluid. Concentrations of testosterone and androstenedione were highest in maternal serum and lowest in amniotic fluid. Cortisol and dehydroepiandrosterone sulphate were found in intrauterine fluids only at the limit of detection but in normal concentrations in maternal serum.CONCLUSIONS Coelomic fluid contains relatively high concentrations of progesterone, oestradiol‐17β and 17α‐hydroxyprogesterone which may be synthesized locally. Amniotic fluid contains lower concentrations of steroids (other than progesterone) than are found in coelomic fluid or maternal serum. Free diffusion of steroids across the amnion appears limited. This may constitute a mechanism to protect the embryo from unwanted exposure to bio