I bled for five weeks (7w - 12w) - heavy, bright red. Then I bled again for a day at 14w2 - again, heavy, bright red flow. This pregnancy is straight after a m/c so obviously I was terrified.
I'm now 20w and it seems everything is fine. We can hear the hb on the doppler, I can feel the baby moving internally, and this morning I felt it externally for the first time. The five weeks of bleeding was a subchorionic haematoma, which is very common (don't google it, there are a load of scare stories out there which horrified me - suffice it to say I've spoken to well over 30 women on various internet forums/real life who've had SCHs, all of which resolved naturally with no ill-effect to the baby). No idea what the one day was, as the SCH had bled out by then. Just one of those things, I guess.
I did exactly what the previous poster did - told my boss that I hadn't planned to say anything so early, but I wanted her to know the reasons for my various unplanned hospital trips, and that it was a threatened m/c. She was absolutely fine, said she'd been worried but didn't want to pry, congratulated me on the pregnancy and told me that she'd bled with both hers and they were both fine.
My DH had to leave work unexpectedly a couple of times - when it first happened we were on a weekend away and had to go to A&E at a different hospital. They insisted I stayed till the scan unit reopened on Monday, so DH couldn't go home either (we had just the one car!). Anyway, he told his boss, not expecting a great deal of sympathy (boss doesn't have time for sickness), only to find his boss's attituted changed entirely. Turned out boss's wife had had 2 m/cs, then her current pregnancy had been threatened several times too, explaining the boss's absence a few times.
I do think you should tell. Hopefully there'll be no further scares, but what if it happens at work? Much better to be able to say to your boss you need to go to A&E immediately and not have to explain then...