Once you know you are pregnant, trying to figure out the ovulation date may seem difficult. Since menstrual cycle in every woman varies, there is no standard day/time after missed period that can help a woman know when fertilisation took place.
If you, however, want to calculate the number of pregnancy weeks you’ve completed, you have to determine your date of conception. And for that, you must find your due date first. There are various methods of calculating due date but Naegele's Rule is the standard method used by obstetricians. Once your EDD is established, estimate your conception date and calculate the pregnancy weeks your have come through.
To calculate pregnancy weeks by the Naegele’s Rule, you need to know the first day of your last menstrual cycle. Once you know that, date back exactly three months and add seven days in the estimated date. Once you have figured your estimated due date (EDD), count back 38 weeks to find the approximate day of fertilisation. Usually, a pregnancy lasts 40 weeks, but some pregnancies may range from 37 to 42 weeks. An obstetrician usually assumes the woman to have a normal menstrual cycle that lasts 28 days before he calculates the due date. If you have irregular cycles, let the obstetrician know.
If you do not know the date of your last menstrual cycle, your gynaecologist will perform an ultrasound scan to determine the gestational age of your pregnancy. The doctor and radiologist will take measurements of the foetus’s legs, arms, head and torso to determine the estimated gestational age. It is on the basis of this examination that the gynaecologist estimates the due date. Once you have your EDD, count back to know your conception date. Ultrasound is usually performed between 8-20 weeks after the last missed period.
[Read: Baby's Growth during Pregnancy]
Some doctors do not calculate the pregnancy weeks from the day of conception but from the first day of the last menstrual cycle. It is because the exact time of conception cannot be determined as ovulation period of a woman varies every month. Therefore, you may be told that you are 4 weeks pregnant according to your last menstrual period (LMP) though the conception would have occurred only two weeks before. In a woman who has a menstrual cycle of 28 days, ovulation happens on or around the 14th day. It is when conception occurs and thus, you miss your next cycle.
To calculate weeks in pregnancy, you can also use conception calculator and determine your date of conception. To find the date of conception on a conception calculator, you should know your cycle length, the first day of your last menstrual cycle and the month in which you menstruated.
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