How is My Due Date Calculated?
The most common way to calculate the date of delivery is Naegele’s Rule, a formula named after the German obstetrician Franz Karl Naegele who created it. Your ob-gyn doctor will ask you for the first day of your last period. He or she then adds nine months and seven days to that date. The end result is a due date about 280 days from the start of your last period. While this may be a simple way to calculate a due date, there are some problems with it. The accuracy of this method depends on:
- Your ability to correctly remember the first day of your last period
- You having regular menstrual cycles
- You not experiencing early bleeding that was not really the start of your cycle
- Your use of oral contraceptives, which could affect the timing of ovulation
Another issue with this formula is that it assumes you ovulated on day 14 of your 28 day menstrual cycle. There have been studies that have found that many women with normal 28 day menstrual cycles don’t always ovulate on day 14. In 1995, researchers analyzed the changes in estrogen and progesterone that normally occur at the time of ovulation in 498 women with normal 28 day menstrual cycles. They found that only 10 percent of these women ovulated on day 14. A study conducted in 2000 found that approximately 30 percent of women with normal 28 day cycles experience fertilization between days 10 and 17; and a 2005 study found that fertilization can theoretically happen at any point during the menstrual cycle. What all three of these studies concluded was that Naegele’s Rule doesn’t reliably correspond with the date of conception, even in cases in which the pregnant woman can accurately remember the first day of her last period. What all of this means is that the delivery date your ob-gyn doctor gives should be used as a guideline of the approximate time your baby will be born, not the exact date.
Can My Due Date Change?
It can. An ultrasound administered within the first 20 weeks of your pregnancy is a much more accurate tool for predicting your due date. As accurate as these tests are, they still have a margin of error of about a week and a half. If your ultrasound due date is within that margin of error, your ob-gyn doctor will probably keep the original due date that was calculated using the date of your last period. If your ultrasound indicates a due date of more than a week and a half away from the original due date, it just may mean that conception occurred earlier or later than what was thought. However, your ob-gyn doctor will usually have you take another ultrasound to see if the baby is developing properly. If that is the case, s/he may change you due date to correspond to the due date predicted by the first ultrasound.
When Does a Past Due Date Cause the OB-GYN to Induce Labor or Perform a C-Section?
The majority of pregnancies last between 37 to 42 weeks. There are a small number of pregnancies that last more than 42 weeks. It is usually at 42 weeks, that there is concern for potential risks including: The placenta may not work as well and the baby will not receive the proper amount of oxygen and nutrients. The amount of amniotic fluid surrounding the baby may decrease to the point where it interferes with the baby receiving oxygen and nutrients. The baby is becoming so large that it may make it too difficult to deliver vaginally. Your ob-gyn will be testing to monitor the baby’s progress. If there is an indication that the baby is having difficulties, s/he may want to induce labor or perform a C-section.
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