Put your pregnancy to the side, the reason you want a payrise is because you have been promoted, have additional responsibilities and have worked hard - right?
So ignoring the pregnancy (as should your boss when you approach him/her), if you think you are worth above the current salary - why do you think this? What do you bring to the role that no one else can/does? How can you sell this idea to your boss?
Do your research: look at similar roles/vacancies in similar Companies (perhaps your competitors) on Reed, [www.totaljobs.co.uk totaljobs]] and print off a number of similar vacancies that show the salary based on your skills and experience.
You could do your own salary benchmarking - have a look on Randstad and Michael Page websites, they normally have free surveys; and I believe the Government produced one recently too (google 'salary surveys')
Pull out previous appraisals, customer/client feedback, emails of thanks from colleagues etc - use this as evidence that you do a good job and that you are valued in the Company.
Then, meet with your manager.
I used to hear quite regularly from certain employees that they were hard done to because of no pay rises in x years. I did a salary benchmark myself and only one of them should have been on more money (the rest were actually way above the 'going rate' for their role). The one who could have had a payrise behaved immaturely and approached it entirely wrong. He should have researched, armed himself with facts, knowledge and had confidence in himself to say "hey, I'm worth more and here's why" - instead, he moaned about the past, was negative in meetings and had an attitude towards me and management that we should just give him a payrise "just because". It isn't the answer and any owner of a business will not just give a payrise "just because they haven't had one in years".
I know it's a completely different story to what you are asking but it is always worth noting that those who ask professionally, do their research and approach the situation positively - do tend to get a pay rise.