Mayra Cabrera, 30, died hours after giving birth to son Zac, who survived, at Great Western Hospital in Swindon, Wiltshire, on May 11 2004.
A nurse wrongly attached the epidural anaesthetic Bupivacaine to an intravenous drip attached to her arm instead of saline solution which she needed to help bring her blood pressure back up.
Mrs Cabrera - who was a nurse at the same hospital - died within minutes from a heart attack caused by the toxic effects of Bupivacaine.
The two drugs had "almost identical packaging" and her life could have been saved if the bags were kept in separate cupboards, the Health and Safety Executive said today.
The Great Western Hospitals NHS Foundation Trust was ordered to pay £75,000 in fines and £25,000 in costs by a judge at Bristol Crown Court. The Trust had pleaded guilty to an offence under the Health and Safety at Work Act.
Passing sentence today, Mr Justice Clarke said: "No one could be unmoved by this tragedy. No one who knew what lay behind it could be untroubled at the systematic and individual fault which this inquiry revealed."
The midwife, who was suspended and is now retired, "could not have read the label carefully or possibly at all", he added, also citing the inadequate drugs storage as a factor in Mrs Cabrera's 2004 death.
An inquest at Trowbridge, Wiltshire, two years ago ruled that Mrs Cabrera was unlawfully killed, also citing the "chaotic" drug storage.
Mrs Cabrera gave birth to son Zac at 8.14am on May 11 2004. She began to suffer a fit and at 10.27am she was certified dead.
The Health and Safety Executive (HSE) said after today's fine that the "heartbreaking case" showed there was no proper system for the storage of the drugs at the time.
A similar mistake in 2001, which did not result in death, had not served as a sufficient warning to prevent the tragedy.
HSE inspector Liam Osborne said: "This was an absolutely heartbreaking case to investigate. Mayra Cabrera needlessly died as a result of comprehensive management failings at board, pharmacy and ward level.
"Had the hospital done something as simple as keeping these completely different but almost identical-looking drugs in separate cupboards, then Mrs Cabrera would not have died.
"It is really important that risks are properly assessed and safe systems put in place to minimise the chance of human error."
The case was made more tragic because Mrs Cabrera used to work as a nurse for the same hospital, he added.
Her widower Arnel Cabrera has now returned to their native Philippines and was raising their son Zac with the help of relatives.
He said in a statement today: "It has been six years since my wife Mayra died and two years since the inquest into her death was concluded. I would like to thank the HSE for bringing this prosecution and I am pleased with its outcome.
"It reinforced the importance of the heath and safety of patients attending hospital and in particular the safe storage of dangerous drugs. Now this case has concluded, I am hoping my young son and I can have some closure and put this terrible tragedy behind us."
Drugs were stored in the same racking system despite having "almost identical packaging", the HSE explained.
Barry Cotter, defending, said the Trust's attitude was far from "cavalier or lethargic", a fact illustrated by its two health and safety awards between 2005 and 2006.
With over one million infusions being carried out each year a "blind spot" would arise from time to time, he added.
Mrs Cabrera's month-long inquest in 2008 was told how the hospital's storage methods failed to meet NHS requirements stating that drugs like Bupivacaine should be stored in locked cupboards separately from intravenous fluids.
There had been two other deaths at hospitals in the UK in the past decade caused by Bupivacaine being administered intravenously, coroner David Masters heard.
Soon after one - that of 74-year-old Philip Silsbury in 2001 at Royal Sussex County Hospital - a memo was sent round Swindon & Marlborough NHS Trust advising that Bupivacaine be kept separately from intravenous drugs to lessen the chance of a mix-up.
At the time the hospital was at its old Princess Margaret Hospital (PMH) site in Swindon, prior to its December 2002 move to GWH.
Stephen Holmes, the now-retired chief pharmacist at GWH, sent the memo on correct Bupivacaine storage around the PMH in 2001.
He was told at the time by staff that this had in fact been the hospital's practice since 1995.
However, these storage standards were not carried over to the new GWH site, with epidural drugs stored alongside intravenous ones.
It was not until after Mrs Cabrera's death that drug storage was brought up to standard.
David Masters, the coroner for Wiltshire, describing the situation as "chaotic", said: "It seems no-one really grasped the aspect of storage at GWH."
He said the move to the new site was "all the more reason why someone should have grasped the issue of storing drugs".
Malcolm Fortune, for the NHS trust, argued that, chaotic drug storage or otherwise, the main blame lay with the person - midwife Marie To - who had attached the bag to the drip without properly checking the contents label - "Bupivacaine: For epidural use only".
Had Ms To gone through all the checks required of her by NHS midwifery protocols, which include getting a doctor to verify the drip bag, Mrs Cabrera would be alive today, he said.
She had denied attaching Bupivacaine to the drip.
The inquest heard there had been three previous non-fatal drug mix-ups involving epidural drugs being attached to intravenous drips at the PMH before the one that caused Mrs Cabrera's death.
One involving Bupivacaine was in 1994, while in 2001 there were two others, one of which involved Bupivacaine.
Alfred Tinwell, 84, died in 2000 at Royal University Hospital, Liverpool, after he was mistakenly given Bupivacaine intravenously.
After the case Lyn Hill-Tout, chief executive of Great Western Hospitals NHS Trust said: "First of all, I want to apologise again to Mr Cabrera, his son, family and friends for the mistakes that happened which led to Mrs Cabrera's death.
"As a result of what happened, a husband does not have a wife and a son does not have a mother.
"We deeply regret this - Mayra's death should not have happened.
"Since her death the Trust has co-operated with six external inquiries and a jury inquest which lasted four-and-a-half weeks.
"We have implemented all of the recommendations from the inquiries and the inquest. The improvements made to the maternity service since Mayra's death led to the Care Quality Commission awarding the hospital the best standard for maternity care.
"Regrettably, we cannot turn the clock back. However, we have learnt valuable lessons and will never again be complacent about patient safety by ensuring it remains our top priority at all times."
