You need to speak to your GP as many don't reccomend taking anti-histomines at all (they are a hormone!)
I was banned from taking any and left with a nasal spray, eye drops and balm tissues to cope with whilst pg with Amy. I figured one hellish summer wouldn't kill me and it was worth it in the end.
I went to the GP and have been told that nasel sprays and eye drops are fine to use in pregnancy and once you are past 12 weeks if you are absolutely desperate & you can't function properly because of the heyfever then you can take Piriton (NOT the none-drowsy type) but it may then make you sleepy! so to weigh up the pros and cons... but I use the nasel spray every day and use my eye drops when they become too itchy! BUT check with your GP!!
I've found something called Haymax that you rub around the nostril and is meant to catch pollen before it reaches the nose! Bit of a fiddle to be honest but its states clearly its safe in pregnancy! I'll let you know if it works!