["Addiction, Volume 121, Issue 6, Page 1520-1529, June 2026. ", "\nAbstract\n\nBackground and aims\nTo measure the proportion of women in New South Wales, Australia, retained in opioid agonist treatment (OAT) for opioid dependence (OD) during pregnancy and examine how this varies according to treatment and maternal characteristics.\n\n\nDesign\nRetrospective cohort study using linked population‐based data, including OAT records, perinatal, hospital admissions, mental health outpatient services and criminal justice data.\n\n\nSetting\nNew South Wales, Australia, January 2004–December 2021.\n\n\nParticipants\nPregnancies resulting in childbirth among women receiving OAT during pregnancy. The cohort included 4472 pregnancies among 2821 women receiving OAT during pregnancy.\n\n\nMeasurements\nWe defined retention as continuous receipt of OAT from the date of OAT initiation during pregnancy or date of conception (whichever came last) until childbirth. We calculated the proportion of women retained in treatment and 95% confidence intervals (CI) overall, by timing of initiation (pre‐conception, first, second and third trimester) and medication type (methadone, buprenorphine) at initiation. We used logistic regression to assess retention variation according to maternal socio‐demographic and clinical factors, including morbidities commonly co‐occurring with OD, stratified by timing of initiation. Data on dosing, other substance use and psychopharmacological medications were unavailable.\n\n\nFindings\nOAT was initiated pre‐conception in 74.8% (3346) of pregnancies, during the first trimester in 11.1% (n = 497), second trimester in 8.8% (n = 394) and third trimester in 5.3% (n = 237). Overall, women were retained in OAT for 84.3% (3771) of all pregnancies. Retention was 87.4% (95% CI = 86.3–88.6) with pre‐conception initiation; 65.8% (95% CI = 61.6–70.0) in the first trimester; 80.5% (95% CI = 76.5–84.4) second trimester; and 85.2% (95% CI = 80.8–89.7) third trimester. Retention was 72.4% (95% CI = 69.5–75.3) with buprenorphine and 87.7% (95% CI = 86.6–88.9) with methadone. Pregnancies delivered during 2019–2021 were less likely to be retained in treatment compared with those delivered during 2004–2006, regardless of the timing of initiation [pre‐conception initiation odds ratio (OR) = 0.49, 95% CI = 0.30–0.79; first trimester initiation OR = 0.29, 95% CI = 0.08–1.07; second/third trimester initiation OR = 0.29, 95% CI = 0.10–0.91). Among women who initiated OAT pre‐conception, retention was lower among those whose first antenatal visit occurred after 20 gestational weeks (OR = 0.68, 95% CI = 0.53–0.86), those in their first (OR = 0.72, 95% CI = 0.54–0.97) or second pregnancy (OR = 0.73, 95% CI = 0.56–0.96) and those who initiated on buprenorphine (OR = 0.31, 95% CI = 0.24–0.40) or in a custodial setting (OR = 0.66, 95% CI = 0.44–1.01).\n\n\nConclusions\nIn New South Wales, Australia, from 2004 to 2021, over 84% of women receiving opioid agonist treatment during pregnancy were retained in treatment until childbirth; however, lower retention in later study years and among women initiating buprenorphine, coupled with recent guidelines recommending buprenorphine as first‐line therapy during pregnancy, highlights the need for ongoing monitoring and targeted support to improve retention among women who are at higher risk of discontinuation.\n\n"]