Morning, Shaggers 
Boy I might have missed it and I forgot to ask, but did you tell your dp/dh yesterday? GL with the digi this morning, if you're doing one?
Ok, I've been very bored googling as to why we don't just get up duffel every month. I think, and this is my conclusion after reading a great deal of woolly opinion based sites, that it comes down to variables.
There are a lot of them and they all need to be right. Your dp's swimmers get made about 70 days before conception (give or take) and so 2 months before js-ing he must not have done anything on that day to affect quality/quantity in that particular deposit. Then you must ovulate, the egg must not get caught at the opening of the Fallopian tube, the egg must be viable and contain the right number of sets of DNA. It must also contain a particular protein to allow the shell to admit a sperm at the right moment. It must float down to meet a sperm. The sperm must be there to meet it and ideally had time to mature (but not so much time that they run out of energy and give up or die). Your CM must be egg white and nutritious enough to support the sperm. Your cervix must be open to admit them in the first place. Then the magic bit must happen and the egg and sperm meet. The divide-y bit must happen correctly (and apparently this is quite hit and miss) and the womb lining must be hospitable, not too thick, not too thin, and the egg must implant properly, releasing the right hormones, and enough of them, to shutdown your ovaries and signal a bfp.
Fucking Christ. That is a LOT of variables, and some of them get more and more variable as you get older. If I am right, the more variables there are, the lower the chance of it occurring as you have to take a 1:2 chance each individual thing will happen (it will, or it will not) and there are 10-15 things which must occur for conception to take place. For all we know some or most of them are occurring every month, it's just that we need them all to happen for a pg to occur.
And you have no control over a lot of these things. You can't control how many pairs of DNA the egg has, for example.
So: the sites seem to say, increase your chances by looking at the things you can control (js-ing at ov time etc).
Does that make sense? It's just my idea.
How are we all this morning?