Fifty-eight gravidas who fell in the high-risk pregnancy category were followed by both single and serial metabolic urinary estriol determinations during their last trimester. Forty-one women (Group A) gave birth to viable babies; 17 (Group B) had babies who died. The concept of decreased urinary estriol output as an indicator of fetoplacental dysfunction has been confirmed, since total estriol levels were almost uniformly lower during the entire trimester in the group of women whose babies died, than in the group who gave birth to viable babies. A new concept of considering estriol concentration (milligrams per liter of urine per 24 hr) has been introduced and may be of value where total estriol output is in a difficult-to-interpret range—“ the shadow zone.”