Your due date is calculated by adding 280 days (40 weeks) to the first day
of your last menstrual period (assuming a 28 day cycle).
Even if you don't have a 28 day cycle, most calculators let you put the cycle
length more than 28 days and up to 45 days. This will give you an
approximate due date. However, if you are not sure of your last menstrual
date or your cycle length is highly irregular, don't worry.
Because, your first scan will surely give you an exact due date based on
the fetus development.
What is the last menstrual period (LMP)?
The LMP is your last menstrual period. It is the first day that your last
menstrual cycle began. That is the date you use when entering the LMP.
Note that your menstrual period and ovulation are counted as the first two
weeks of pregnancy. If you deliver on your due date, your baby is actually
only 38 weeks old, not 40.
What is the difference between an online conception
calculator and the “wheel” conception calculator that a doctor
uses?
There is no difference between the two. The doctor uses the first day of
your last menstrual cycle to estimate due date and conception date in much
the same way the online conception calculator works.
All results are based on the input, so you need to know the accurate first
day of their last menstrual cycle to achieve the best and most reliable
results.
Please remember that your due date is only an estimate. Every pregnancy
is unique and your baby will come when it's ready. Be sure to talk to your
health care provider about your due date.
On average only 5% of births take place exactly on the estimated due date.
Most are born within a week either side of the estimated due date. A
normal pregnancy can last anywhere between 38 and 42 weeks.
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-Guest Author on familygynaec.com