Calculating Pregnancy Due Date, Due Date Calculator in Pregnancy
The estimated date of delivery (EDD) is calculated as 40 weeks (280 days) from the start of your last menstrual period. Conception usually occurs two weeks after the last period and the duration of the pregnancy is 38 weeks (266 days) from conception. But this only applies if:
- You have a regular four-week cycle
- The first day of the last period is certain
- There has been no bleeding since your last period
- No contraception was being used (if you conceived after coming off the pill then the last menstrual period is not reliable for calculating your dates)
My menstrual cycle is regular but isn't 28 days long. Can my EDD still be calculated from my periods?
If you have a menstrual cycle that is longer than 28 days - say, 35 days - but regular, then the estimated date of delivery can be calculated by adding 41 weeks to the first day of your last period. Conversely, if you have a regular 21-day cycle, the estimated date of delivery is calculated by adding 39 weeks to the last day of the menstrual cycle. This is because ovulation and conception occur almost 14 days before year next period is due, regardless of the length of your cycle. If you have an irregular cycle or a cycle longer than 35 days, the EDD cannot be estimated reliably from starting date of your last period.
How do doctors confirm the stage of my pregnancy?
It is critical to have a good assessment of the stage of your pregnancy in case complications arise. It is best, therefore, to confirm your dates by an ultrasound scan. Especially where there is uncertainty about your menstrual dates, ultrasound is very reliable at dating the pregnancy in its early stages. Convention has always related gestation to the first day of the last menstrual period, so for women who know the date of conception, such as those undergoing assisted conception.
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