A pregnancy calculator can work forward or backwards depending on what you know and what you need to find out. With a pregnancy calculator you can, for instance, use the date you conceived to determine the due date for your baby as well as the dates when you will enter your second and third trimesters. This might be especially important in cases of rape or incest or any other circumstance in which the mother might be considering terminating the pregnancy.

You can use a pregnancy calculator in reverse as well. If you know, for example, how far along you are in your pregnancy due to a pelvic exam and other symptoms the calculator can determine the date of conception. This would be especially important if paternity is in question.

For women who are irregular and so don't really know if and when they actually missed their menstrual period, the pregnancy calculator can still be a useful tool for determining conception, the current stage of pregnancy as well as the mother's due date. To use the pregnancy calculator in this way all you need to know is the first day of your last menstrual period.

Most women will experience some fluctuation of timing and symptoms from those indicated by a pregnancy calculator. This is because the calculator assumes a 28 day menstrual cycle and some women are not that regular. But a pregnancy calculator can be a reasonably accurate guide as you prepare for your baby's birth and for the changes in your body and your life that will occur as a result.

Knowing, for example, when you should be concluding your first trimester is encouraging for those who are suffering the nausea and vomiting of morning sickness. This queasiness almost always disappears at the end of the first trimester. Knowing you're about to enter your third trimester is crucial if you're thinking of aborting your pregnancy. Most women choose to have an abortion during their first trimester, so this is an important timeframe to be aware of. The abortion is far easier and the pregnancy less advanced and less obvious. Knowing the onset of your third trimester is important as well because it is also the time when your baby drops and you can breathe again but want to stay close to a bathroom at all times.

There are many female-focused or medical care Web sites that offer free pregnancy calendars. Some are more advanced and versatile than others, but virtually all provide you with a breakdown of what you can expect from your body during the various stages of pregnancy and what your body is doing and how she or he is evolving from one week to the next. With some of these calculators you must know your due date, while others let you indicate your date of conception. Still others only require that you are aware of the start of your last menstrual period.

Knowing what symptoms and body changes to expect at what point helps you understand that those aches and pains and minor symptoms aren't cause for concern, and lets you know the danger signals to mention to your physician should they occur.