question: The start of my last menstural period was Oct 25 2010. Lasted about 7 days. According to that, I m supposed to be 12 weeks today (Jan 15 2011). However, I went for my dating ultrasound yesterday and was told I was only 10 weeks and 6 days. Therefore they couldn t do the Nuchal Translucency scan. My question is, what exactly is gestational age? Does this calculate when the baby was concieved? And if so, the baby would have been concieved the last day of my period (Unlikely, I thought?) I m confused as to which age I should be using. If my gestational age was different, shouldn t everybody else s be? I hear people say I was only off by 2 or 3 days , but wouldn t that mean they also concieved during their period? Please help.