Background: Undocumented pregnant women constitute a vulnerable group of people who lack equal access to pregnancy care. Previous research has shown that undocumented migrants encounter difficulties in accessing health services, the onset of prenatal care is delayed, and women have an increased risk for infectious diseases. The aim of this study was to describe the use of maternal health care services and the obstetric outcomes of undocumented women in Helsinki, in addition to comparing the results with all pregnant women in Finland.Methods: The study was a retrospective register-based study consisting of data collected between 2014 to 2018 from the electronic medical records of the public maternity clinic and maternity hospital in Helsinki, Finland. The study population consists of 62 individual pregnancies of undocumented women. The results of the study were compared with national data on parturients and deliveries (N=47,274 women) and with prenatal screening tests for infectious diseases (N=51,447 [HIV, HBV], N=51,446 [syphilis]) (relative risk, RR with 95 % CI).Results: The majority (91 %) of the undocumented women attended public prenatal care. However, four women received no prenatal care and three women were denied access to care. Undocumented women entered prenatal care later and had fewer visits compared with all pregnant women. The majority (71 %) of the undocumented women received inadequate prenatal care as the number of visits was less than eight. Of the study population, 5 % (3/61) tested positive for HIV, 3 % (2/61) for HBV, and 2 % (1/62) for syphilis. The prevalence of HIV (RR 62, 95 % CI 20–195), HBV (RR 16, 95 % CI 4–63), and syphilis (RR 20, 95 % CI 3–145) were higher amongst undocumented women compared with all pregnant women.Conclusions: Undocumented women entered prenatal care later than recommended. Most women received inadequate prenatal care and some of them did not receive prenatal care at all. The prevalence of infectious diseases was higher and the coverage of prenatal screenings deficient amongst undocumented pregnant women.