Interesting question - although I would argue that if there's a need for extended leave for birth mothers, there is also a need for extended leave for dads/partners too. 

The earliest that a mum can start maternity leave is the 11th week before the expected week of birth. If baby is born prematurely and mum hasn't yet started maternity leave, it will start automatically the day after the birth. There is no flexibility over the start date, as it is compulsory to be off work for the first two weeks after childbirth (four weeks if you work in a factory). 

Once maternity leave has started it runs for up to 52 weeks. This can be extended with up to 4 weeks' unpaid parental leave (per child). 

The situation for Dads/partners is arguably worse: Paternity leave covers up to two weeks (which don't have to be consecutive) after the birth or adoption of a child.  

Leave can start on any day of the week but must usually be completed within 56 days of the actual birth. If the baby is born early, leave must be completed within the period from the actual date of birth up to 56 days after the expected week of birth.

This does mean that parents of premature babies have a longer period after the birth in which to take paternity leave, but with many preemies in hospital for weeks after birth, it would be really tough for families whose employers stick to the statutory minimum entitlements. 

There is the option of shared parental leave, but this relies on only one parent being off work at any one time.  

Of course, many employers do exercise a much greater degree of flexibility, whether through enhanced formal policies or more informal means of support for employees going through the trauma of a child born prematurely. 

Perhaps it is time that the Gov't looked again at family leave entitlements, including the very low (2%) take-up of parental leave in the UK?