Background Active screening of only pregnant and lactating mothers (PLMs) excludes other mothers of reproductive age susceptible to undernutrition. Our analysis evaluated if mothers presenting with wasted children were more likely to be undernourished themselves. Methods The observational study enrolled mother and child dyads presenting to an outpatient facility in Mogadishu, Somalia, between November 2019 and March 2020. Trained nurses recorded lower extremity oedema for children aged 6–59 months, parity and gestational status for women aged 19–50 years and age, access to care, height/length, mid-upper arm circumference (MUAC) and weight for both. Weight-for-height z-score (WHZ) for children and body mass index (BMI) for mothers were calculated using standard procedures. Wasting was defined as WHZ <−2, MUAC <12.5 cm and/or presence of oedema for children. Undernutrition was defined as MUAC <23 cm for PLMs and BMI <18.5 kg/m2 for neither pregnant nor lactating mothers (non-PLMs). Four multivariable linear regression models were fit to evaluate maternal anthropometric indicators (BMI or MUAC) given child anthropometric indicators (MUAC or WHZ), adjusting for maternal age, parity and gestational status. Results A total of 93.6% (2142/2288) of enrolled dyads met inclusion criteria. Wasting was observed among 57.5% of children; 20.2% of pregnant mothers, 20.0% of lactating mothers and 7.95% of non-PLMs were undernourished. Models suggest significant, positive associations between child and maternal anthropometrics; a one-unit increase in WHZ and a 1 cm increase in child MUAC were associated with 0.22 kg/m2 (95% CI 0.22 to 0.24) and 0.19 kg/m2 (95% CI 0.16 to 0.21) increases in maternal BMI, respectively, and 0.20 cm (95% CI 0.18 to 0.22) and 0.24 cm (95% CI 0.23 to 0.25) increases in maternal MUAC, respectively. Adjusted R2 values were low (range 0.06–0.10). Conclusions Undernutrition among non-PLMs illustrates the importance of expanding screening. However, while significant, the strength of association between mother and child anthropometrics does not support child nutritional status as a screening tool for identifying at-risk mothers.