How can I determine my due date
A pregnancy generally lasts for 40 weeks from first day of your last menstrual period, or 38 weeks from conception. If you know the date of your last menstrual period, or the date of conception (some women do!), and your cycles are regular, you should be able to determine your due date sitting at home.
Based on LMP:
If your cycles are regular and 28 days long, then you can determine your due date by adding 9 months and 7 days to the first day of your last menstrual period (LMP). For example, if your LMP began on September 5, then your expected due date (EDD) would be June 12 of the following year.
Based on date of Conception:
If your cycles are 28 days long and you know your date of conception, you can determine your due date by first subtracting 7 days, and then adding 9 months. However, this method will probably not work for most women unless they have had intercourse only once during the fertile period of that cycle and remember the date.
How does the doctor determine my due date
The doctor will examine your abdomen at each of your regular monthly visits. One of the things that the doctor will be checking for is the size and growth of the baby. Although this method cannot pinpoint a specific date for delivery, it can certainly tell the doctor if the due date estimated by either the LMP or the date of conception is in the correct range or not. For instance, sometimes the LMP may predict the due date as April 15th. But based on an abdominal examination the doctor may feel that the baby is very big (growing too fast), he/she could revise the due date and move it up by 5-10 days (or even more) to perhaps April 8th.
One of the reasons that the doctor asks for frequent ultrasound scans, especially in the last few months, is that they can determine the development of the baby and provide a more accurate due date. This method is superior to other methods because the doctor is able to examine and measure each of the baby's organs on ultrasound screen. These measurements can then help in determining the size of the baby, i.e. stage of fetal development, and consequently the due date. It is possible that each of your ultrasound scans gives a different due date. The date computed from your last ultrasound scan is likely to be the most accurate. Again, remember that these dates are simply estimates, and therefore are only indicative of the actual delivery date.
Can different methods give different due dates
Yes. Each method will most probably give a different due date, but most of the time, they will vary only by a couple of days. However, if the woman has an irregular menstrual cycle, or her cycles are very long or very short, then the LMP method will not be accurate. In this case, an ultrasound scan will be most accurate.
Can I also plan to have my baby on a particular day
Yes, you can try to plan your baby for a particular day. However, remember, your menstrual cycles and ovulation times would have to cooperate! If you want to have your baby on a particular day, you can subtract 38 weeks from the planned due date. If it is easier to work in months and days, you should first add 7 days from your planned due date, and then subtract 9 months.