BACKGROUND: While Doppler ultrasound screening is beneficial for women with high-risk pregnancies, there is insufficient evidence on its benefits and harms in low- and unselected-risk pregnancies. This may be related to fewer events of abnormal Doppler flow, however the prevalence of absent or reversed end diastolic flow (AEDF or REDF) in such women is unknown. In this systematic review, we aimed to synthesise available data on the prevalence of AEDF or REDF. METHODS: We searched PubMed, Embase, CINAHL, CENTRAL and Global Index Medicus with no date, setting or language restrictions. All randomized or non-randomized studies reporting AEDF or REDF prevalence based on Doppler assessment of umbilical arterial flow>20 weeks’ gestation were eligible. Two authors assessed eligibility and extracted data on primary (AEDF and REDF) and secondary (fetal, perinatal, and neonatal mortality, caesarean section) outcomes, with results presented descriptively. RESULTS: A total of 42 studies (18,282 women) were included. Thirty-six studies reported zero AEDF or REDF cases. However, 55 AEDF or REDF cases were identified from just six studies (prevalence 0.08% to 2.13%). Four of these studies were in unselected-risk women and five were conducted in high-income countries. There was limited evidence from low- and middle-income countries. CONCLUSION: Evidence from largely observational studies in higher-income countries suggests that AEDF and REDF are rare among low- and unselected-risk pregnant women. There are insufficient data from lower-income countries and further research is required.Doppler ultrasound can be used during pregnancy to determine how well blood is flowing through the umbilical cord. When this blood flow is restricted, absent or even reversed, the health of the baby can be threatened. Poor umbilical blood flow can lead to a baby experiencing growth restriction. If the flow is absent or reversed, the baby may die. In this review, we aimed to determine how often pregnant women experience abnormal umbilical flow during pregnancy, in particular the occurrence of absent or reversed flow. We were interested in how often this occurred in women who had a singleton, low-risk pregnancy (i.e. women without significant medical, obstetric or fetal complications of pregnancy). We found 42 studies reporting on over 18,000 women, mostly from high-income countries. Across all studies, 55 women experienced absent of reversed blood flow in the umbilical artery, all of which occurred in just six studies. However we found limited evidence from low- and middle-income countries, where rates of growth restriction and preventable stillbirth are quite high. Further research on abnormal umbilical blood flow in pregnant women in low- and middle-income countries is required.Additional file 1: Appendix S1. PRISMA checklist.Additional file 2: Appendix S2. Search strategy.Additional file 3: Appendix S3. Included studies.Additional file 4: Appendix S4. Quality assessments.UNDP/UNFPA/UNICEF/WHO/World Bank Special Programme of Research, Development and Research Training in Human Reproduction (HRP), Department of Sexual and Reproductive Health and Research, and World Health Organization.https://reproductive-health-journal.biomedcentral.comObstetrics and Gynaecolog
