The number of weeks that a woman is pregnant is medically known as the gestational age or menstrual age. Gestational age is commonly represented in terms of weeks elapsed. It is essential to record the gestational age during your doctor’s visit to know what developmental milestones to expect from the growing baby.
Gestational age can be calculated using different methods. Using a calculator to know how many weeks pregnant you are is the most convenient method. It also helps to calculate the due date of delivery.
Calculating the age of pregnancy is not only important to evaluate the normal pregnancy milestones, but also to detect growth defects, if any, in the baby. In some cases, the growth of the baby does not correspond to the gestational age. In such cases, which also includes other complications of pregnancy, gestational age helps in deciding the due date of delivery.
A normal pregnancy extends for 40 weeks. Calculation of how many weeks pregnant the woman is can be done using different methods as follows:
1. Pregnancy calculator based on last menstrual period (LMP) is used for calculating the gestational age. The calculation is based on the consideration that conception takes place on the 14th day of a 28 day menstrual cycle, which is the day of ovulation. This method can prove faulty in cases where periods are irregular, which causes the ovulation date to shift. This is particularly true in conditions like polycystic ovarian disorder. Some women may be unable to recollect the exact date of the last periods too.
2. The first day of the last menstrual period is also used to calculate how many weeks pregnant you are by your due date. Doctors generally use the Nägele rule for calculating the estimated date of delivery. In this rule, 7 days are added to the first day and 9 months to the month of last menstrual period. An obstetric wheel is sometimes used to calculate the gestational age and expected date of delivery. This wheel has an outer wheel with calendar dates and inner sliding wheel with days and weeks of gestation. This method is also not very accurate due to frequently occurring irregularities in the monthly periods
3. The period from the date of conception up to the due date of delivery is called conceptional age or fetal age. The conceptional age is 2 weeks lesser than the gestational age. The term of pregnancy considered while calculating gestational age using date of conception is 38 weeks. As it is impossible to know the exact day of conception, calculating gestational using the date of conception is a faulty method.
4. Calculating how many weeks pregnant after IVF or in vitro fertilization is comparatively easier. This is because the date on which the embryo is transferred in the mother’s womb is considered as the first day of gestational age.
5. Ultrasound scan or USG is the most accurate calculator of gestational age. The reference values for the scan findings of crown-rump length (CRL), biparietal diameter (BPD) or head circumference and the femur length (FL) of the growing baby are used for calculating the gestational age.
6. Other methods of calculation exist but are not commonly used to calculate gestational age. These include measuring the height of the growing uterus (symphysis-fundus height) and sensation of baby movements, which occur around 20 weeks of pregnancy.
Nowadays, a free online calculator to know how many weeks pregnant a woman is also available for accurate calculation of gestational age.
In case of a difference of 7 days in the gestational age calculated between the LMP and USG method, the gestational age calculated by the LMP method is considered more reliable. This is called the 7-day rule. In the 10-day rule, the USG scan derived gestational age is considered more reliable when the difference is more than 10 days.
Gestational age calculated using the LMP and USG methods are used in combination to avoid large errors.
Disclaimer: The information in the article is not intended or implied to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of your doctor.