OBJECTIVE: To determine the frequency, severity, prognosis, and patterns of carpal tunnel syndrome (CTS) in pregnancy. DESIGN: Descriptive retrospective chart review using the Rochester Epidemiology Project medical record diagnostic indexing system to identify patients with new CTS occurring during pregnancy from 1987 to 1992 at our institution. SETTING: Obstetrical practice, where two thirds of pregnant women in the county receive primary obstetrical care. PATIENTS: Women pregnant during 1987 to 1992 who had a new diagnosis of CTS. Women with pregnancies at other dates or women who had CTS with onset before or after pregnancy were excluded. OUTCOME MEASURES: Age, underlying medical problems, gestation interval, weight gain, number of pregnancies, presenting symptoms, onset and duration of symptoms before diagnosis, trimester of CTS diagnosis, treatment and response, and results of electrophysiologic studies are described. RESULTS: Of 10,873 pregnant patients receiving antenatal care for 14,579 pregnancies, 50 (.34%) fulfilled the inclusion criteria. Their mean age was 30.5 +/- 4.0 yrs. Twelve patients (24%) were primigravid. Mean weight gain was 12.1 +/- 5.7 kg. CTS was diagnosed most frequently during the third trimester (n = 25, 50%). Symptom onset, when recorded, occurred with even distribution during each trimester: first, n = 11 (32%); second, n = 11 (32%); third, n = 12 (35%). For 37 patients in whom symptom duration was recorded, duration before diagnosis was 9.3 +/- 9.0 weeks. Paresthesia (88%) was most often bilateral (68%), and 67% of patients had pain. The Tinel sign was present over the median nerve at the wrist in 95%. Only nine patients had nerve conduction studies performed. During pregnancy, 37 women were treated nonsurgically with wrist orthoses, steroid injections, or both. Of treated patients for whom follow-up data were available, 25 of 26 improved, and 4 of 26 required surgery. Thirteen women had no treatment during pregnancy; three underwent surgery in the postpartum period. All 7 women in whom conservative treatment failed who underwent surgery had resolution of symptoms. CONCLUSION: These results represent the frequency and patterns of clinically significant CTS in a large population of pregnant women. CTS severe enough to warrant treatment occurs infrequently in pregnancy and generally resolves spontaneously postpartum or responds to conservative treatment.