They determine it by how big the baby is. I havent had a period in two years because my son is breastfed, so when I got three positive at home tests they brought me in did a vaginal ultrasound measured the baby and by statistics of how big a baby should be at certain times they put your baby in a category and vulah you have an estimated date. Its not always perfect tho, babies wil come when they want. My first baby had a due date of march 25th by my period, and by ultrasound was april 16th. three weeks different, which is a huge gap. it should only be a week or so. But he was born a very healthey boy in size, weight and breathing very well, on march 16th. so just know your baby will get here when they are ready. I hope my answer helped.
at 4:37 PM on Apr. 28, 2009