If you have been patiently or perhaps not so patiently waiting for a pregnancy test to come up positive, your very next question after that positive is about when you think your baby is due to be born. You will get a very accurate due date once you have an ultrasound, and your doctor will also calculate due date for you at your first appointment, but those are both things that you have to wait for. You can do a little detective work on your own to come up with a due date that will be pretty close to what your doctor or ultrasound will eventually tell you.
One thing that will determine how close you get when you calculate due date on your own is how well you know your own cycle. If you have what doctors consider to be a regular cycle of 28 days, you can make a good guess about your ovulation date and your due date. If you are one of those women that writes down when you have your period, you can get a very close date to the one your doctor is going to give you. However, those with longer or even shorter cycles can also make an educated guess.
While you know that most pregnancies are about 40 weeks long, you are actually only pregnant for 38 weeks. Two weeks are counted towards your due date that are technically when you are not pregnant. In order to calculate due date, you have to go back to the first day of your last period, not the day of ovulation. These two weeks before you ovulate are considered part of your forty weeks. Most are confused as to why these two weeks are counted even though they are not pregnant at the time. The egg was maturing, but it was not yet fertilized.
If you have longer cycles, you can be off by a week when you calculate due date on your own. However, if you know that you ovulate three weeks after the first day of your period which is also the first day of your cycle, then you can adjust accordingly. In that case, if you went with the common pregnancy wheel, your due date will be a week off. The ultrasound will correct this, but you can correct it on your own if you have kept track and know you ovulate on day 21 rather than day 14, for example.
Those women that feel when they ovulate can make a very accurate guess when they calculate due date on their own. They can count ahead 38 weeks from that day to get a pretty close guess as to the date their doctor or ultrasound is going to give them, no matter when they started their last period. If you have missed a few periods, you may not remember accurately when you last started a cycle, but if you are a few months in, your appointment will come soon enough and then you will know for sure.