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How Accurate Is My Due Date?


Calculating your due date can be a tricky matter. Adding to the difficulty is that every pregnancy is different, and that it is sometimes difficult to determine the actually date of conception. However, there are standard methods used to help a woman know when her baby will be due.

“Nagele’s Rule” was developed in the mid-nineteenth century by Dr. Nagele. Dr. Nagele calculated that the average pregnancy lasted 266 days from the date of conception, or 280 days from the last menstrual period (referred to as LMP). This is where we get the figure that pregnancy lasts for 40 weeks – 40 weeks is exactly 280 days.

Recent research has suggested that the 280 day rule may not be exactly accurate. They suggest that the average pregnancy for first-time Caucasian moms, for example, is 288 days from the LMP. A variety of factors, including ethnicity, the age of the mother, prenatal nutrition, and maternal weight can pull or push this number in a variety of directions.

If you are unsure of the date that your last menstrual period began, this can throw off your due date. In addition the factors listed above can affect the duration of your pregnancy.

Other methods that may give you a more accurate due date can include:

– Measuring your Basal Body Temperature and your Lutenizing Hormone (LH) to determine the date of conception. These methods allow you to know exactly when you are ovulating. By adding 266 days, you can find your due date.

– An experienced health care provider may be able to help determine your due date by using ultrasound technology to determine the gestational age of your baby, and thereby calculate your due date.

– If you know for certain when conception occurred, you can fairly accurately predict a due date.

– One of the most reliable ways to calculate your due date is to track HCG levels in early pregnancy. For a normal pregnancy, HCG levels double roughly every 2 1/2 days. Your health care provider may be able to perform these tests to help determine your due date.

– Other measurements can give a more general due date. For example, as the baby’s heartbeat can generally be heard through Doppler at around 9-12 weeks, your due date can roughly be calculated by this. The size of your baby, measured by the fundal height, can give some indication of when you should be due.

Even if you are certain of the date of conception, a due date prediction is still a prediction. Your pregnancy may or may not be exactly average in length.



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