The majority of pregnancies last around 40 weeks (or 38 weeks from conception), which is why many doctors will estimate your due date to be counted 40 weeks from the start of your last period. While this method is universal, it’s interesting to note than only 5 per cent of women go on to deliver on their estimated due date via this method. And increasingly it’s just as normal to go into labour a week or two before or after your pregnancy due date. If you experience regular menstrual cycles, there are two ways in which to calculate your due date (otherwise know as the EDC – your estimated date of confinement).

Naegel’s Rule

Naegel’s rule is based on a simple calculation: add seven days to the first day of your LMP (last menstrual period), then subtract three months.

  • As example, if you LMP was December 1st 2014
  • Add seven days (December 8th 2014)
  • Subtract three months , but forward the year if required
  • The EDC would be September 8th 2015

The Pregnancy Wheel

The other way to calculate your pregnancy due date is with a pregnancy calculator that’s virtually at your finger tips. A pregnancy wheel is a method many health care providers use, for ease of interpretation, as it’s very straightforward to estimate your due date. You or your doctor can easily locate the date of your last menstrual period, as you line up the date with the indicator and the wheel will then display your due date(usually accurate to within a couple of days). Inhealth.ie stocks the FertilAid Pregnancy Plus Pregnancy & Ovualtion Calendar Wheel .

What If I’m Unsure Of My LMP?

Your health care provider can arrive at your pregnancy due date via other methods if you are unable to accurately give the dates of the first day of your last menstrual period. Both a physical exam or ultrasound study can help to determine your due date accurately.Ultrasound measurements are more accurate for estimating gestational age early in pregnancy because early development occurs according to a fairly regular schedule, please note that when a doctor performs an ultrasound, he or she will write a report on the findings including two estimated due dates. Pregnancy tests such as The Clearblue Pregnancy Test, Double Check & Date can both confirm your pregnancy and the date of the conception.

What If My Periods Are Irregular?

For many women the menstrual cycle can be quite irregular or consistently last longer than the average 28 days. If this applies to you, you are still able to use a pregnancy calculator with adjustments. In general terms the second half of a woman’s menstrual cycle is consistent and lasts around 14 days.

If your menstrual cycle is longer you can calculate your pregnancy due date by adjusting this method. Work out as accurately as you can when you ovulated and using this adjusted LMP you can determine the EDC that way. Some pregnancy wheels will allow you to enter the date of conception rather than the LMP conception (union of egg and sperm which happens within 72 hours of ovulation). To trick your pregnancy calculator wheel you would subtract 2 weeks from the day you ovulated to mark as your LMP and then view the date where the line crosses for your EDC. Some pregnancy wheels will allow you to enter the date of conception rather than the LMP. In trying to regulate your periods and take control of your fertility window, know your LMP with the Persona Monitor, designed to allow you to control your conception and contraception naturally it works to monitor hormone levels, giving you knowledge and control.

Why Would My Doctor Change My Due Date?

Your doctor may alter your pregnancy due date if the size of your fetus differs significantly from the appropriate size for your calculated age of pregnancy. If there is a history of irregular periods, or if conception occured during oral contraceptive use, or if the pelvic examination suggests a gestational age difference from that indicated by the LMP. In these instances your doctor will order an ultrasound which will enable him to measure the length of the fetus from one end to the other in order to ascertain the crown-rump length. During the first trimester this measurement is normally in line with the actual gestational age plus or minus seven days, as the pregnancy progresses and the rates of fetal growth begin to vary from pregnancy to pregnancy, these later measurements via ultrasound cannot accurately be used to predict gestational age. For more information on all aspects of pregnancy and conception discover the brilliant Zita West and her book ‘Fertility & Conception’ the complete guide to getting pregnant. Aimed at couples that are planning to start a family, from those who have already experienced problems conceiving to those who have only recently decided to have a baby, Fertility & Conception is loaded with advice and up-to-date information on maximizing fertility.

For the full range of Fertility and Pregnancy aids available on Inhealth.ie click on Fertility.

22nd Mar 2018

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