Theoretical basis of measurement of estriol during pregnancy. It was clarified that the feto (adrenal and liver)-placental unit plays an important role in the biosynthesis of estriol during pregnancy. The theoretical basis of the usefulness of estriol estimation in maternal urine as the indicator of fetal viability is therefore established, and a simplified estriol assay (LAIR-3 minutes method) was developed. As to the steroid values in the amniotic fluid during pregnancy, high level of progesterone (P4) in the first trimester and prominent rises of DHA-S and estriol values near term were shown. Furthermore, transference of amniotic DHA-S to the mother through the amniotic membrane was demonstrated by the in vivo study using deuterium labeled DHA-S given in the amniotic fluid. In vivo study using deuterated pregnenolone sulfate (P5-S) given in the maternal circulation demonstrated that maternal P5-S was partially used as the precursor of placental P4. Therefore, it is suggested that the precursor of placental P4 is mainly derived from the feto-placental side rather than the maternal one. Changes of steroidal environment near term. Steroid hormone assay by gas chromatography mass-spectrometry using deuterium labeled compound as internal standard was developed, and (fetal adrenal) steroid values in maternal blood were measured. DHA-S, 16 alpha-OH- DHA-S and estriol values increase in the prepain and labor period and P4 and 20 alpha-OH-P4 values decrease during labor. In vivo study using deuterium labeled DHA-S given to the fetal side in the perinatal period demonstrated that DHA-S originated from fetal adrenal transferred to the mother through placenta without being subjected to the aromatisation. The elevation of uterine sensitivity to oxytocin in the perinatal period is closely related to both the increases of DHA-S and estriol levels, and the decrease of P4 values in the maternal blood. As to the hormonal factors of hypothalamo-posterior pituitary system, the levels of estrogen stimulated neurophysin (ESN) and oxytocin in the maternal blood elevated parallel with the increase of estrogen level in the prepain period. The administration of DHA-S (100mg, twice a week) in the perinatal period accelerate the maturation of uterine cervix with concomitant augmentation of DHA-S concentration and increased proline hydroxylase activity in the cervical tissue.(ABSTRACT TRUNCATED AT 400 WORDS)