The serum levels of pro-type I collagen peptide (PIP), pro-type III collagen peptide (PIIIP) and type IV 7S collagen (7S collagen) were measured in 41 women in various gestational weeks of pregnancy (5-41 weeks). The PIP level did not change during pregnancy, being 450 +/- 380 mg/ml before week 37 and 448 +/- 280 mg/ml after week 37. The PIIIP level also did not change being 0.55 +/- 0.21 and 0.59 +/- 0.22 U/ml before and after 37 weeks. The PIIIP level increased slightly but not significantly after week 37. The 7S collagen level, however, increased significantly (p less than 0.01), being 5.1 +/- 0.8 and 6.6 +/- 0.8 mg/ml before and after week 37. The correlation coefficients with the Bishop score were 0.22 for PIP, 0.42 for PIIIP, and 0.72 for 7S collagen. These data suggest that serum 7S collagen closely reflects collagen degradation in the cervix and so should be a good marker of cervical maturation.