My midwife said that I was large for dates. In fact, she estimates that I am six weeks further along than I thought. I am confident of my dates. She has assisted at over 1,000 births and is currently teaching in the nurse-midwifery program at the University of Minnesota. There are several explanations why a doctor or midwife might over or underestimate gestational age.
Women can experience bleeding during early pregnancy, which they believe is a period.
Then, when they come in for their exam, the uterus is larger and they are due sooner than expected. A uterus may also be larger, of course, if there is a multiple gestation, or in rare cases when a molar pregnancy or pelvic tumor is present. Or, although somewhat uncommon, the woman conceived and then miscarried and then conceived again. In that case, the woman might be erroneously counting from the previous pregnancy dates. I had a client a few weeks ago who had regular cycles but once in a great while, she would skip a period.