Let's be realistic.
You just can't!
If I say you still can concentrate, when you are already seeing tweety birds flying on top of your head and having that sourish feeling down your throat, I would be lying.
but I do suggest you take more short breaks.
I dont't know what you are working as.
can I presume its office work?
if it is,
take more moments of lean back on your chair, close your eyes for 2-3mins.
Do it like every half hour.... or even 15mins... it really depends on how serious you are feeling....
but THIS you'll have to see how 'understanding' your boss is too.
for me, when I was pregnant, I was running bazaars and pushcarts, so I did that too... and I'm glad that my neighbouring vendors help me to look after the stall while I 'rest' or else, I think many stuff would have been stolen from my stall...