The cost of living is a difficult one as it depends on where you live, whether you buy or rent (if you buy you pay property taxes, rentals you don't etc) , what your lifestyle is etc! I find it more expensive here for groceries and utilities as the latter there is no competition so you can't shop around for the best deal, like in the UK
There's been a couple of discussions on this board before, and a couple of us were in the same ball park on monthly costs for a family in the NY / NJ area, all in it was needing take home pay of 8-10k a month which included mortgage or rent to be comfortable (altho I know some people would baulk at this)
If you have a DC already, I'd be looking to factor in the cost of a private school or moving to an area with a decent public (same as a UK state school) school as there's a definite correlation between cost of housing and school district
Regarding getting pregnant, I think it might be a shock in regards to how little you get here. Unless you work for a more enlightened company (and its changing but slowly) 4 months leave is pretty unusual.
You don't get SMP or even actually maternity pay - getting pregnant and giving birth is classed as a short term disability 
In NY and some other states, there is a law that you can take 12 weeks for family medical leave (FMLA) covering pregnancy and still have a job to come back to, however this is unpaid leave. Your pregnancy and paid leave is covered by short term disability insurance, the coverage of which depends on your employer and insurance plan.
I got a heady 8 weeks paid (not 100%) leave as I had a section. I then took 4 weeks unpaid leave on FMLA and then another 12 weeks unpaid sabbatical leave. However I'd been with my company 7+ years and I negotiated with my manager. This left me wide open tho to getting fired whilst I was off, with no comeback (didn't happen, but it can)
However, this is all due to change next year with the intro of a paid leave law in NYC. Only covers 8 weeks paid leave tho at 55% pay, so nit great if you already qualify for that - here's more info for you www.ny.gov/new-york-state-paid-family-leave/paid-family-leave-how-it-works
All of this ^ tho will depend on you being with the firm for at least 26 weeks (I think) before getting preggers