Pregnant women with pre-existing medical conditions face increased risks of complications, and pregnancy can exacerbate these conditions. However, Australian evidence on the prevalence and impact of such conditions during pregnancy is limited. A retrospective review of 18,000 births at Nepean Hospital, a tertiary NSW Level 6 Maternity service, from 2019 to 2024, was conducted to analyse the prevalence of pre-existing medical and mental health conditions using booking-in data. This data was cross-matched with over 40,000 antenatal clinic appointments in 2024 to assess the relationship between maternal comorbidities and outpatient service demand. The study found that 68% of women had at least one pre-existing medical or mental health condition, with 19% having three or more. Those with multiple conditions required significantly more antenatal appointments, with a stepwise increase in service demand corresponding to the number of systems involved. The most common conditions were a mental health diagnosis (31%), anaemia (21%), asthma (20%), and migraine (11%). The conditions requiring the most clinic appointments included type 1 diabetes and chronic hypertension. Women managing a mental health condition with medication attended, on average, two more antenatal visits compared to those without pre-existing conditions. These findings highlight the high prevalence of pre-existing conditions in pregnancy, the increased demand for specialist antenatal care, and support current models of integrated obstetric and physician care. They also inform service planning and resource allocation in tertiary maternity settings.