Conception dates can be confusing to estimate with or without known Last Menstrual Period dates.

Conception is typically calculated using the first day of a woman's last menstrual period(LMP). This can be confusing often times due to not knowing LMP or having a 'period' after conceiving. Often times, a woman will spot or have a light period shortly after conceiving. This is typically due to the embryo attaching to the uterus which causes bleeding. This is often mistaken as a light or spotty period. Often times a conception date will be estimated based upon LMP, then confirmed with the first sonogram which shows fetal growth. This measurement is typically given in Gestational Age (age since LMP) versus Fetal Age (age since conception). There is typically 2 weeks difference in Gestational versus Fetal age (i.e. 7 weeks gestation, 5 weeks fetal age). Gestational age should always be larger than fetal age.Combined with LMP information, an accurate conception and due date can be given. Sonogram readings have a margin of error. At 8 weeks +- up to 6 days, at 20 weeks +- up to 10 days, beyond 24 weeks +- 2 weeks. If you are unsure about paternity and have had more than one partner in a 10 day timeframe - a DNA test will be necessary to determine paternity with certainty.

http://www.medcalc.com/pregwheel.html#?314,526 is a good pregnancy wheel - same as used by doctors. Week 0 is first date of LMP, week 1-2 is most likely conception time frame.http://www.healthboot.com/pregnancy/due-date-calculator/ is a good online due date calculator. Due dates are a 'best guess' based on a typical 40 week (from first day of LMP) pregnancy. Typically 38 - 39 weeks after conception date. Normal time between menses is 28 days, some women can range from 21-40 which can change ovulation therefore changing conception date possibilities.