There is generally no risk in working up to the final month of your pregnancy. You may decide to work right up until you deliver, but by this week you are highly likely to find it increasingly tiring. If you are feeling exhausted and you think you might want to start your leave sooner, speak to your boss without delay. Hopefully, he or she will be understanding, but realize that if you start your maternity leave early, you may have to come back to work sooner (see Your Rights and Benefits). But if you are struggling to cope with the discomforts of late pregnancy, the decision to take sick leave during the last four weeks may be the right move for you. Talk to your doctor and your employer as soon as you make this decision.
Another desirable option is to work flex-time, so that you can travel to work at a less busy time and, if it's possible with your type of employment, work from home sometimes to cut out some of the commuting.