Talk to a senior science colleague; get to know the school policy on sex education and find out where else the pupils (officially) encounter the subject.
Help. They've started giggling already
This will stop them: involve the whole class in setting the rules - no question to be laughed at and only use proper names for body parts.
OK, but how far should I go?
All pupils have a right to learn about reproduction, but this is not sex ed so leave the complex bits (relationships, contraception, STDs and hormones) until they are older and stick to the biology for now.
Do I have to draw them a diagram?
How about creating a "classwomb" where the children role play, acting the part of the sperm swimming through the uterus to meet the egg, which is being passed down the "ciliated cells" (represented by fingertips) lining one oviduct. At the moment of fertilisation, 2 Become 1 by the Spice Girls makes great background music.
What if someone else has beaten me to it?
You'd be amazed by some of the misconceptions they might still have. Ask them to estimate how much "blood" is lost in a period by choosing between beakers of various volumes of liquid. Lots of them will choose the litre jar - a 50ml beaker is about right.
What's the worst that can happen?
Be prepared for anything. Encourage pupils to post questions anonymously in a box and filter them. Omit inappropriate ones and you could even add questions you wish they had asked
Jeremy Airey is professional development leader at the National Science Learning Centre, York