The actual date depends a lot on your ovulation cycle. You need to conceive 40 weeks before you want your baby to be born. You would need to conceive in June 2010 for the baby to be born in March 2011. 16th June to the 23rd March is exactly 40 weeks. So, to have a baby between 23-27 March 2011, you should aim for conceiving between 16th-20th June 2010. Of course, your chances are greater if you're ovulating during this time. Good luck.