About when would this baby have had to have been concieved?

Well if in reality the first day of her last period was March 4th than she could of conceived anywhere between the 15th and 22nd of March. Usually a woman will ovulate 14 days after the first day of her period and can get pregnant for several days starting the 14th day. The 14th day having the highest possibility for conception, and the later days having smaller and smaller changes. The reason why her due date keeps changing is because the gyno calculates the pregnancy starting from the first day of her last period when in all reality you don't actually ovulate until 14 days after the first day of your last period. So even though they say, "Oh your 6 weeks pregnant", your baby in reality about 4 weeks old. It all comes down to if she really did have her period on the 4th of March or not.