I've never known anyone that can be that 'clinical' about getting pregnant - surely not many people can plan things with that accuracy?
What if the baby is due in the Autumn and arrives early, or due in the Summer and arrives late?
Anyway, there are advantages and disadvantages for all sorts. If you hang around the "parents supporting their children through University" threads, you'll see very good financial reasons for leaving a 4 year gap between dc.
. I guess this also applies for paying for childcare when they were little.
OTOH it makes it difficult to entertain them all as age gaps become bigger - types of holiday, days out, even eateries, etc. I have a friend who has 3 in 3 consecutive school years and she loved the fact that they were all at one activity at similar times - say cubs or Brownies - but I liked the fact that my older dc had generally left before their sibling started, so they had their own 'space' and weren't always lumped together as being 'dc1's sister'.
Two yr gaps tend to give you exam stress at GCSEs and A-levels, but only if all dc go through the system "typically" without any reason not to.
I found that my two who were closer in age, didn't really get on unitil they were about 16 and 14, as dc1 always felt a bit 'threatened' by dc2's abilities and he didn't cope well with the competitiveness, getting on much better with his younger sibling who wasn't 'competing' with him but my nieces who are 1 school yr apart have always got on famously - so that can go either way too.
When there are things you don't particularly enjoy - be that having to get in the water at the swimming baths with them, or organising birthday parties for them, it does seem to go on for ever a long time if there are big gaps. I'd hate to start all that again once it had stopped for a while. That said, if there are bigger gaps then the older ones can be more helpful in keeping an eye on the little one for 10mins while you nip out, etc.
So many different reasons for each way. 