OBJECTIVES: To summarize the evidence on typical and perfect-use effectiveness of fertility awareness-based methods for avoiding pregnancy during the postpartum period, whether breastfeeding or not. STUDY DESIGN: We conducted a systematic review of studies published in English, Spanish, French, or German by November 2021 in MEDLINE, EMBASE, CINAHL, Web of Science, and ClinicalTrials.gov. Abstract and full text reviews were completed by 2 independent reviewers. Study inclusion: at least 50 subjects who enrolled prior to experiencing 3 cycles after childbirth and were using a specific fertility awareness-based method to avoid pregnancy; unintended pregnancy rate or probability calculated; postpartum amenorrheic and postpartum cycling individuals analyzed separately; and prospectively measured pregnancy intentions and outcomes. Outcomes were abstracted and study quality was systematically assessed by 2 independent investigators. RESULTS: Four studies provided effectiveness data for 1 specific fertility awareness-based method among postpartum individuals. Of these, there were zero high quality, 1 moderate quality, and 3 low quality for our question of interest. Typical-use pregnancy probability for the first 6 cycles postpartum for Marquette Method users was 12.0 per 100 women years (standard error [SE] not reported) and for Billings Ovulation Method users ranged from 9.1 (SE 3.9) for non-lactating women <30 years old to 26.8 (SE 4.6) for lactating women