Prevalence of abnormal umbilical arterial flow on Doppler ultrasound in low-risk and unselected pregnant women: a systematic review

Table 1 Characteristics of included studies

Study design	N	%
Case–control study	7	16.7
Cross-sectional study	12	26.2
Cohort study	19	45.2
Randomized controlled trial	4	9.5
Country income level
High-income countries	24	57.1
Upper-middle income countries	13	31.0
Low-middle income countries	4	9.5
Low-income countries	0	0.0
Multiple^a	1	2.1

^aOne study was a multi-country study conducted in Brazil (upper-middle income country), Kenya (low-middle income country) and the UK (high-income country)

ISSN: 1742-4755