The due date for pregnancy is calculated using the Naegels rule or the pregnancy wheel. Both methods calculate the due date based on the date of the last menstrual period. An ultrasound done in the first trimester also helps in estimating the pregnancy due date, states Healthline.
In the Naegels rule, the calculation of the due date for pregnancy is done by adding seven days to the first day of the last menstrual period. From this figure, three months are subtracted, and finally, if required, the year is altered to the subsequent one, explains Healthline.
Most often, health care providers use the pregnancy wheel to determine the approximate due date. For this, the date of the last menstrual period is identified and lined up with the indicator. The pregnancy wheel then displays the probable delivery date. In women with a 28-day menstrual cycle, the pregnancy wheel proves to be quite accurate. For women with long or irregular period cycles, the pregnancy wheel is used by considering the date of ovulation to determine the adjusted date of last menstrual period or the precise conception date, Healthline states.
If a woman is unsure of the date of her last menstrual period, if her periods are irregular or if conception occurred even with oral contraceptive use, ultrasound examination is performed in the first trimester of gestation. Ultrasounds measure the length of the fetus, which correlates well with the gestational age during the first trimester and tends to be accurate within seven days, reports Healthline.