In general, pregnancy due date is calculated from the first day of the last menstrual period (LMP). However, it is very difficult to find out the exact day of conceiving the baby. Hence, it is considered that a woman ovulates and gestates 14 days exactly after the first day of her last period. Assuming this condition, a baby is already 14 days old at the time of conception. Determining Pregnancy Due Date The most popularly used pregnancy due date calculator is Naegele's Rule, which is a mathematical formula. In developing this numerical relation, Dr. Naegele assumed that the menstruation cycle lasted for 28 days, and ovulation occurred on the 14th day of the cycle. He also opined that the average duration for human gestation was 266 days from conception or 280 days from the LMP. According to this rule, the due date is determined by adding seven days to the first day of the LMP, and subtracting three months from the sum result. For better understanding, let's take an example to determine when the baby is likely to arrive. If the first day of the last menstrual period was on January 1, 2009, then the estimated due date falls on October 8, 2009. However, in some cases, the estimated due date turns wrong. This may be due to faulty date of the last menstrual period, in case of incidents like implantation bleeding. Since the symptoms of implantation bleeding resemble normal menstruation, the date of LMP may be confusing. In such a situation, the date of LMP may be taken wrong. An accurate date is essential for determining the correct due date. When it comes to the accuracy of the due date calculated by using Naegele's Rule, one of the major concerning factors is the ovulation period. Though he assumed that the ovulation occurred on the 14th day of the menstrual cycle, it is not true for all women. Some other factors that were not taken into account while developing the rule include ethnicity, prenatal care, prenatal screening, nutrition, and risk factors. A more reliable way to determine the pregnancy due date is the ultrasound test. This method is effective in case of any uncertainties regarding menstruation. For accuracy, ultrasound should be taken during the first half of pregnancy (within 12 - 13 weeks of pregnancy). In addition to this, a combination of factors like luteinizing hormone, basal body temperature, fetal heart tones, quickening (first fetal movement that the woman feels), and fundal height (uterus measurement) should be considered for calculating the pregnancy due date. Nevertheless, it is to be noted that not every pregnancy lasts for exactly 40 weeks. The due date is just a prediction as to when the baby is likely to be born. A pregnancy is considered to be normal, if the gestation period lasts between 38 to 42 weeks. Normally, a pregnant woman gives birth to a baby three weeks before the due date, or two weeks after the due date. As per the statistics about 5 percent of pregnant women give birth on the due date, and about 80 percent give birth within 10 days of the due date. In case of any problems with the baby and/or the mother, the obstetrician may suggest inducing labor, either before or after the due date. Childbirth prior to the normal gestation period is referred to as premature birth. Also, if the due date extends more than two weeks after the actual due date, the obstetrician may recommend for labor induction. Disclaimer: This Buzzle article is for informative purposes only, and should not be used as a replacement for expert medical advice.