Hi Jenny

At the beginning of pregnancy, hCG is only around in small amounts, so the more concentrated the urine the better. You can just have nothing to drink and not go to the loo for four-six hours in the afternoon and, for hCG, there will be no difference. It's just easier to be able to drink and use the loo, that is why the first urine is recommended. And you can always test right now and if you get a BFN wait a couple of days and then do a FMU.

A lot of hormones follow a daily rhythm, so when you have your urine (or blood) tested can affect the reading - for example, testosterone (in both men and women) is more concentrated in the blood at 9am and lower in the afternoon. But hCG isn't circadian so you can test at any time, but the more concentrated the urine the better chance the test has of picking it up.

As for how much the concentration differs, keep some pots in the loo. Look at the colour of your FMU compared to one later in the day - the FMU can easily be 3-4 times more concentrated, depending on how much you're drinking in the day.