OBJECTIVE: To determine if a false [ + ] T18 MMST (Maternal serum alpha fetoprotein, [MSAFP] ≤ 0.75 MOM, estriol < 0.60 MOM, and human chorionic gonadotropin, [hCG] ≤ 0.55 MOM) indicates increased risk for obstetrical complications or is related to maternal weight. STUDY DESIGN: We accessed our genetic data base, containing MMST results, fetal karyotypes, and pregnancy outcomes from patients seen in the prenatal diagnosis clinic from 1993 to 1996 (n = 5376, mean maternal age = 32.5 ±6.7). In the MMST, only MSAFP had been adjusted for maternal weight. We selected all patients with a [ + ] T18 screening test (n = 119, 2.2%) and all patients with a normal MMST (n = 3933, 73%) and compared pregnancy outcomes. RESULTS: A [+'] T18 screen identified 42% (5/12) of T18 fetuses. Compared to those with a normal MMST, women with a false [ + ] T18 screen were heavier (181 ±49 Ibs. vs 161 ±38, p = 0.0002), and younger (30 ±7 years vs 32 ±7 years, p = 0.0008). Weight adjusting estriol and hCG in addition to MSAFP reduced the screen [ + ] T18 rate by 32% (from 2.2% to 1.5%) and reduced the number of amnios required per case detected (from 1/24 to 1/11), but did not change the T18 detection rate. Compared to women with a normal MMST, women with a false [ + ] TIH screen were at no increased risk to have a preterm birth, low birth weight, spontaneous abortion, or neonatal demise. CONCLUSIONS: A false [ + ] T18 screening test does not indicate increased risk to develop pregnancy complications and may he related to higher maternal weight. Weight correction of all 3 analytes reduces the number of false [ + ] T18 screening tests by one third.