This systematic review of systematic reviews investigated the effectiveness of lifestyle weight management interventions for postnatal women. We systematically reviewed Medline (PubMed), Embase, CINAHL Plus, The Cochrane Library, and Scopus from 2000 until January 2018, to identify systematic reviews of randomized controlled trials that evaluated the effectiveness of behavioural lifestyle interventions for weight management in postnatal women. Results were summarized both descriptively and statistically using a mega meta-analysis of data from randomized controlled trials included in previous systematic reviews. Nine systematic reviews met our inclusion criteria. Overall the reviews concluded that lifestyle interventions involving physical activity and/or dietary changes resulted in a reduction in postnatal weight. Results from the overall mega meta-analysis confirmed this finding with a mean difference of −1.7 kg (95% CI, −2.3 to −1.1). Findings for subgroup analyses gave mean differences of −1.9 kg (95% CI, −2.9 to −1.0) for combined diet and physical activity interventions, −1.6 kg (95% CI, −2.1 to −1.2) for physical activity-only interventions, and −9.3 kg (95% CI, −16.5 to −2.1) for diet-only interventions (one study). Heterogeneity varied from 0% to 68%. Interventions involving lifestyle interventions appeared to be effective in reducing weight in postnatal women, although these findings should be interpreted with some caution due to statistical heterogeneity. KEYWORDS intervention, postnatal, systematic review, weight 1 | INTRODUCTIONObesity is a key contributor to many chronic co-morbidities including type 2 diabetes, cardiovascular disease, 1 stroke, and a number of types of cancers such as colorectal and breast cancer. 1-3 These conditions can be life threatening, detrimental to quality of life, and expensive to treat.The obesity epidemic is affecting all populations, including women of reproductive age. Recent national surveys reported that approximately 66%, 56%, and 58% of women in the United States of America, Australia, and England, respectively, either have overweight (body mass index [BMI] between 25 and 30 kg/m 2 ) or obesity (BMI over 30 kg/m 2 ). 4-6 This means that most women already have overweight when they become pregnant. A period of notable weight gain for many women occurs during and after pregnancy. 7,8 Studies have reported that among women who have a healthy BMI prior to pregnancy, 30% have overweight 1 year after giving birth. 9,10 Of women who have overweight prior to conception, 44% have obesity 1 year after giving birth, while 97% of women with obesity prior to pregnancy remain so at 1 year postnatally. On average, women gain about 14 to 15 kg during pregnancy, and at 1 year after birth, 5 to 9 kg is retained. 8,11 Some women are able to return to their prepregnancy weight after childbirth, but the amount of weight women retain postnatally varies considerably, and many women never lose all of the weight gained during pregnancy. [11][12][13][14][15]