I know it can be very confusing, but there is a simple explanation for the difference between your calculated weeks of pregnancy and how far along you are in your pregnancy according to conception date. The best way to explain this is with some basic info about the menstrual cycle.
Pregnancy is calculated based on the average menstrual cycle, which lasts 28 days. The first day of menstrual bleeding marks the first day of the menstrual cycle. This is called your LMP, or last menstrual period. During the first few days of the cycle, menstrual bleeding (your period) occurs as the uterus sloughs the lining that developed in the previous cycle. Afterwards, uterine lining (endometrium) grows again in preparation for possible conception.
Approximately two weeks into this cycle, ovulation occurs. The few days before and after this event is when you are most fertile. If during this time you have intercourse and sperm is released, fertilization(and thus, pregnancy) can occur. After fertilization and throughout pregnancy, the endometrium does not slough (meaning no menstrual bleeding) because it is now supporting the developing baby. If fertilization does not occur, the endometrium disintegrates and menstrual bleeding follows, signaling the beginning of a new cycle.
When calculating the weeks of pregnancy, we include those two weeks from the first day of the LMP to the approximate date of ovulation and conception. Because the first day of your LMP is the easiest part of the cycle to notice and record, that is where the beginning of the menstrual cycle and your pregnancy is measured from. It is about halfway through the average woman’s menstrual cycle that she ovulates and therefore conceives. So, just as your ovulation calculator indicates, your date of conception should coincide with right about the time your husband came home from deployment.
Whew…no need to worry there! So your little embryo may be only four weeks old, but you are six weeks pregnant. It sounds a little silly, but you aren’t really pregnant for about the first two weeks of your pregnancy. Finally, your estimated due date (EDC or EDD) is then calculated as 40 weeks from your LMP... And yes, this does mean that baby is actually only 38 weeks old when born.