1. Because it is countable. Like most nouns, 'labour' can be either countable or uncountable, depending on precisely what is meant when we use the word. In your sentences, 'labour' is in fact countable. A long labour, a short labour, a difficult labour - all of these refer to the specific periods leading up to specific births, so they are countable. A woman could say, for example, 'I've had three easy labours'. In this comment, 'labours' refers to the specific and personal episodes she has experienced, rather than 'labour' as a general phenomenon. 2. It wouldn't mean exactly the same thing.A mother can have a long and arduous labour followed by an easy delivery. Conversely, it's possible to have an easy labour followed by difficult delivery. NB Apologies to anyone who's used to US spellings. I realise you've probably never seen quite so many superfluous letter 'u's in one place before. Just blame Noah Webster for having them removed in the 1780s!