ImportanceAlthough new parents’ mental health is known to decline, less is known about changes in therapy attendance, especially among military service members.ObjectiveTo investigate changes in therapy attendance among new parents and by parental leave length.Design, Setting, and ParticipantsThis cohort study of US Army and Navy service members from January 1, 2013, to December 31, 2019, compared parents’ monthly therapy attendance with matched nonparents’ across childbirth and compared mothers’ weekly therapy attendance before vs after returning to work. Eligible monthly sample members included service members with first births from January 1, 2014, to December 31, 2017, and 12 months of data before to 24 months after birth and nonparents with 36 months of data. Eligible weekly sample members included mothers with first births from January 1, 2013, to June 30, 2019, and data from 12 months before to 6 months after birth and nonparents with 18 months of data. Data analysis was performed from July 1, 2023, to January 15, 2024.ExposureThose exposed to parenthood had no prior children, acquired a dependent younger than 1 year, and, for mothers, had an inpatient birth. Unexposed matches did not add a dependent younger than 1 year.Main Outcomes and MeasuresMonthly counts of mental health therapy sessions and any therapy sessions (weekly).ResultsThe monthly sample included 15 554 193 person-month observations, representing 321 200 parents and matches, including 10 193 mothers (3.2%; mean [SD] age, 25.0 [4.9] years), 50 865 nonmother matches (15.8%; mean [SD] age, 25.0 [5.0] years), 43 365 fathers (13.5%; mean [SD] age, 26.4 [4.8] years), and 216 777 nonfather matches (67.5%; mean [SD] age, 26.4 [4.8] years). The weekly sample included 17 464 mothers. Mothers went to 0.0712 fewer sessions at 1 month post partum (95% CI, −0.0846 to −0.0579) compared with 10 months before birth. Fathers went to 0.0154 fewer sessions in the month of birth (95% CI, −0.0194 to −0.0114) compared with 10 months before. Parents with preexisting treatment needs had larger decreases in treatment. Weekly therapy attendance increased by 0.555 percentage points (95% CI, 0.257-0.852) when mothers returned to work from 6 weeks of leave and 0.953 percentage points (95% CI, 0.610-1.297) after 12 weeks of leave.Conclusions and RelevanceIn this cohort study of new parents, therapy attendance decreased around childbirth, especially among parents with prior mental health needs and mothers with longer maternity leaves. These findings suggest that more accessible treatment, including home visits or telehealth appointments, is needed.