Recent studies have suggested that a fifth of pregnant women suffer from high blood pressure, and a small percentage of those go on to see this level continue or even increase.
There are a few things you can do to combat this increase. Firstly, try and fit in as much exercise as you can (with your doctor's approval). This will help your blood pressure and keep your weight under control, which can sometimes be a factor in high blood pressure.
Avoiding salt is also worth doing, and cut out any smoking. Experts also recommend you try and keep as stress-free as possible - not always so easy with a newborn!
Have a look at the NHS guidelines on lowering your blood pressure.
One supplement you might like to try is called help: blood pressure, from Works with Water (£15.79 for a starter pack).
As with all supplements, double check with your doctor before taking, especially if you're breast feeding or or any medication.