Pregnancy's are dated from the first day of your last menstrual period, so when you find get a positive pregnancy test your already around 4 weeks pregnant. So with your first pregnancy, you were 4 weeks when you found out, so your 'start date' (first day of your last menstrual period) was 19 December. Unfortunately you lost that baby at four weeks, but you got pregnant again straight away! I hope this helped, sorry for your loss but best of luck with your new pregnancy