im 44, and have 5 dc's from 21 down to 5....Im desperate for one more to complete my family....all of the 5 (plus a darling angel daughter that died in utero at 14 weeks) took me 3 months or less to conceive....it ook me 2 years to catch with #6 ony for me to loose it at 9 weeks in may.
I never told anyone I was TTC because of my age and already having 5, but had my big cheesy 'american stylee' pregnancy announcement all planned out, where I was going to make sure that no-one was under any illusion this was an accident, and that it was planned.....at my age, 'accidents' can be viewed as carelessness in the contraception department, or being 'unable to keep my legs shut' (some people can be so judgemental and cruel!)
I also had a huge thing to post on FB to counteract comments like
'is your telly broke' (no i just have sex during the ad breaks)
'Are you mad' (quite possibly, yes but the meds help enormously)
'Dont you have enough already/youre just greedy (not really....i plan to be a VERY difficult pensioner, so my kids need all the reinforcements they can get)
and so on.
as its taken me so long to conceive, I totally understand the bitterness/bafflement towards those who fall pregnant (like my SIL who accidently (and i do mean total 'accident') fell pregnant with number 4, and said to my hubby, she was devastated and didnt want another, but now is all happy and joyous all over facebook, like shes always wanted it and it was perfectly planned etc.
I was very uncharitable about it to my o/h and said at her age (37) she should be able to monitor her own contraception properly or keep her legs shut......oh....wait...... 
but on the other hand, i have 2 sisters, one who has never been able to have kids, either naturally OR by IVF, and one who had one child then got secondary infertility due to going through a sort of menopause in her mid 20's, and I always dreaded telling them that I was having a baby, so much so that none of my other pregnancy announcements have been particularly big, flashy or even joyous moments because of this.
but...back to the o/p....for something thats supposed to be SO natural and how the world works and how we evolved....its pretty bloody tricky when you really have only a 24 hour window in every month, to actually 'catch'....makes you wonder how the human race continues with those odds!