Polly Ross had suffered from hyperemesis gravidarum during her pregnancies. She had to be hospitalised and put on a drip which compounded her mental health problems.

A mother who killed herself while suffering from postnatal depression died as a result of a “very serious failure” that allowed her to leave a mental health unit unchaperoned, a coroner has ruled.

Despite having made multiple attempts to kill herself, 32-year-old Polly Ross was allowed to leave the Westlands mental health unit in Hull at about 8.30am on 12 July 2015, telling nurses that she was going to buy cigarettes. She was hit by a train at 11.10am and died instantly.

Speaking at the end of a four-day hearing, coroner Prof Paul Marks said he could not rule that Humber NHS foundation trust had been guilty of clinical neglect, but said the decision to allow her to leave the unit “had a direct causal effect” on her death.

Her mother, Jo Hogg, who was previously employed by the trust as an occupational therapist, thanked the coroner for conducting a “frank and fearless examination” of the circumstances surrounding her daughter’s death.

She said the trust had failed her daughter when she had needed their help the most and that care for women with postnatal depression in the region was “appalling”. She said that mental health services were “not joined up in a way that pays close regard to the complex needs of patients”.

The court heard how Ross, who ran a translation business in Paris before moving back to east Yorkshire in August 2012, had suffered from the extreme form of morning sickness, hyperemesis gravidarum, during both her pregnancies in 2012 and 2014. The condition has received media attention after it was revealed that the Duchess of Cambridge suffered from it during her pregnancies.

The condition caused Ross – who was described as “staggeringly intelligent” – to be hospitalised and put on a drip, which was said to have compounded her mental health issues. The inquest was told that she developed “drug-induced psychosis” after taking cannabis to relieve her symptoms and that when she asked to be admitted to a specialist mother and baby unit in Leeds, she was turned down.

In February 2015, the linguist was sectioned after a breakdown and her children were taken from her care. Over the coming weeks and months she regularly expressed suicidal thoughts and attended A&E on multiple occasions having self harmed or taken an overdose.

In a statement read to the court, Ross’s aunt Emma May, who acted as her carer after she was first sectioned, said she was certain that the few times her niece had left her home since February “were times she attempted to take her own life”. She said: “I cannot understand how she was allowed to leave the unit to buy her own cigarettes the morning she died.”

Giving evidence to the inquest, Dr Robert Kehoe, a Bradford-based consultant psychiatrist, said that while the overall standard of Ross’s care had been good, there were two serious failures on the part of Humber NHS foundation trust.

“One: there was a failure to clarify and state a plan for what should occur in the situation of a patient requesting to leave the unit,” he said. “Two: the effective decision to end the period of 15-minute observations allowed her to leave the unit at around 8.40am that day.”

Ross’s observations had been increased from once an hour to once every 15 minutes on 10 July after a ligature was found in her room. She was not sectioned at the time of her death, but Kehoe said there was “no logic” in increasing her observations only to allow her to leave the unit unescorted.

In a statement, Humber NHS foundation trust said: “We would like to offer our sincerest condolences to Polly’s mother, aunt, other family members and friends for their tragic loss. The thoughts of everyone associated with the trust continue to be with them at this sad time.

“We would also like to offer an unreserved apology to Polly’s family and friends and acknowledge that there were omissions in her care prior to her death on 12 July 2015. The trust acknowledges Prof Marks’ conclusion regarding the circumstances surrounding Polly’s death and has fully implemented all of the recommended improvements highlighted by our investigations.

“The trust will continue to reflect and learn and seek to continually improve the services we provide to patients.”

In October 2015, Marks ruled that Humber NHS foundation trust was guilty of neglect in the case of Sally Mays, 22, who killed herself after being turned away for inpatient mental health care. The same year, a coroner in Bristol raised concerns about mental healthcare for new mothers after 30-year-old Charlotte Bevan jumped off a cliff clutching her baby girl following a “chain of failures” by medical staff.