You need to inform your boss that you are pregnant and will be taking maternity leave at least 15 weeks before your baby is due.

When is best?
As soon as possible, but wait until you're sure you're pregnant and everything is normal (a month and a half to two months is a reasonable amount ot time to wait).

How?
Your midwife will provide you with a Mat B1 form which your employers will need in order for you to benefit from maternity rights and maternity leave.
You need to provide your employer with confirmation of your pregnancy, your due date (week) and the date you intend to start taking maternity leave and Statutory Maternity Pay.

Maternity leave and maternity pay
Women are now entitled to 52 weeks maternity leave according to new laws (April 2007). It doesn't matter how long you've been working for a company or how many hours you work: as long as you give the correct notice you are entitled to full maternity leave.

Statutory Maternity Pay (SMP) is now extended to 39 weeks. You receive six weeks' pay at 90% of your salary and SMP for a further 33 weeks at a maximum of £123.06 a week, even if you decide not to return to work. To qualify for SMP you must have been: working for the same employer without a break for at least 26 weeks into the 15th week before the week your baby is due and earning an average of at least £84 a week before tax.

The new laws also include a Keeping In Touch day agreed upon by you and your employer, on which you come into work whilst on maternity leave to keep up to speed with what's going on. You may also work an additional nine Keeping In Touch days without your maternity leave coming to an end. You must take at least two weeks' Statutory (compulsory) Maternity Leave (four if you work in a factory).