Maternity units already criticised by NHS inspectors still have "significant staffing concerns", the Care Quality Commission (CQC) said.
The CQC visited Basildon, Southend, and Broomfield hospitals in Essex in July and August after issues were raised.
Basildon hospital's maternity unit was rated inadequate last year due to problems with leadership and staffing.
The Mid and South Essex NHS Foundation Trust said 40 additional midwives had been hired in the last week.
A full report has not been sent to the trust, but a summary of the CQC's findings in maternity units across its hospitals revealed staffing was having an impact on services, the Local Democracy Reporting Service said.
The summary said: "Across all clinical areas it was clear that there were significant midwifery staffing concerns which were impacting on the service and staff.
"Where possible, this was mitigated using non-clinical staff and flexible working by senior leads.
"Midwives were frustrated by the staffing shortages and felt they were not being heard by ward managers."
Last year, improvements were called for at Basildon Hospital's maternity unit following a string of safety incidents, including one in which a mother bled to death shortly after giving birth.
Ron Woodley, deputy leader of Southend Council, raised concerns the merger of the three hospitals was having a negative impact.
The independent councillor said: "The government said it would increase money to the NHS but they need to look at the whole management structure.
"To have one chief executive running three hospitals is unworkable and doesn't give a focal point of management."
A Trust spokesman said: "We are really pleased to say we have around 40 midwives who have joined the trust this week, which will begin to help fill the gaps in midwifery rotas across our sites."
The CQC inspections also noted a faulty lift in Southend's urgent and emergency care unit that meant there was a 20-minute journey for patients being transferred to intensive care.
A trust spokesman said the the hospital was waiting for the delivery of parts to fix it.