Julie Welsh, 28, had just time to hold and kiss her newborn son before suddenly falling unconscious. Medics battled for an hour and a half to revive her.
Fiance Pete Brown was left devastated when their efforts failed, and he wept as he told Derby and South Derbyshire coroner Dr Robert Hunter how Julie had kissed baby Isaac moments before suffering a cardiac arrest.
Julie suffered a fatal amniotic fluid embolism, the inquest heard.
Fluid surrounding her son in the womb had entered her blood stream and caused an allergic reaction which is so rare it affects only 18 in a million women. In up to 80 per cent of cases, the condition is fatal.
Pete, 30, said: 'Julie seemed fine so I asked if she wanted to see Isaac and I left her for a minute, then took Isaac over to see her.
'She saw him, smiled and made a comment about his hair and things like that. Then her eyes just shut.'
Julie, from Long Eaton, Derbyshire, had been admitted to Royal Derby Hospital on April 5 after going into labour.
However, despite many hours in labour, the decision was made to carry out a Caesarean as the baby was in distress.
The surgery was a success, but minutes later she fell unconscious and was pronounced dead at 12.20am on April 6.
Dr Hunter said: 'One minute you're on cloud nine with the baby and within an hour-and-a-half you're in deepest despair.
'I can't begin to imagine what that is like.'
Recording a verdict of natural causes, Dr Hunter said the deadly condition resulted from 'the natural process of childbirth'.
He said: 'Unfortunately, it cannot be predicted, there are no tests or factors that can be assessed.
'Furthermore, there's no specific treatment that can be offered and therefore it is not preventable.'
Julie's father Les Welsh, 50, added: 'Whenever I kiss Isaac I will do it twice - once for me and one from his mum. She is a star missing from our sky now.'