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How to Calculate Your Baby's Conception Date

Katlyn Joy |24, April 2015


Sometimes you want to be sure about your conception date. Of course, the primary reason for most women to need the answer is to estimate the due date of the baby. But there can be other issues at play, such as when a woman has had sex with two men since her last menstrual period, for instance. Or perhaps she had irregular cycles and needs to pinpoint a more specific date in a timeline to accurately plan for the baby's birth.

Traditional methods assume standard timing. Using a calendar type system, you figure a due date by adding 280 days to last menstrual period, or equaling 40 weeks. That's great if you have regular, 28 day cycles and ovulate predictably on day 14.

In September 2014, the American College of Gynecologists and Obstetricians along with the American Institute of Ultrasound in Medicine and the Society for Maternal-Fetal Medicine, released new guidelines for physicians and other health care providers for accurately estimating due dates for pregnant women.

Knowing the true due date is crucial in many obstetrical decisions, from whether or not a baby is post-due and needs labor needs to be induced, or if steroids will need required for baby to speed lung maturation. It will also help physicians to know if a fetus is growing as it should.

This new guideline was actually the first time ACOG has issued a statement on the topic of how to accurately date a pregnancy.

The guidelines give precedence to the findings of an early ultrasound over other methods, including date of last menstrual period or LMP. Should the date of LMP and the early ultrasound conflict, the ultrasound should set the date as it is considered more accurate.

An early ultrasound is one performed at less than 14 weeks. It is estimated that 40 percent of pregnancies would have a change of due date is using the ultrasound to date the pregnancy.

The precise breakdown goes something like this: should a woman's LMP and the ultrasound conflict by more than 7 days and the ultrasound was performed before 14 weeks, the due date should be changed to reflect the dating by the ultrasound. Should the ultrasound have been performed earlier than the 9th week, and the due date is off by 5 days according to LMP, the due date should be adjusted to the one indicated by ultrasound. The reasoning is that the earlier the ultrasound, the more accurate it is believed to be.

Ultrasounds are more accurate early because up until the 13th week of pregnancy, all babies grow at roughly the same rate. After that, individual differences take over.

Information yielded from second and third trimester ultrasounds cannot be give such credence as they are nowhere near as accurate this late in pregnancy in dating a pregnancy.

Only 5 percent of babies are born on their actual due date, so a date cannot be taken as a precise prediction but rather as an estimate of a range. By adhering too closely to a precise date, too many babies may be induced leading to a much higher rate of Cesarean birth.

Dating Ultrasounds

A dating ultrasound is done pretty much the same as a regular ultrasound, except with a full bladder and with a trans-abdominal transducer. However, using a trans-vaginal transducer with an empty bladder will be a bit more accurate.

Whenever there is any doubt about the dates, a dating ultrasound may be performed. These conditions giving rise to doubt include:

  • having irregular cycles,
  • last menstrual period a mystery to woman,
  • unusual cycles,
  • only recently stopped taking hormone contraception pills,
  • a recent miscarriage,
  • the woman is still breastfeeding,
  • or there was implantation bleeding.

Dating scans can also yield other important information such as the location of the pregnancy as in regards to placental issues, the number of fetuses and/or sacs, the size of the fetus, detecting the heartbeat, and if any unusual features are found in the uterus such as fibroids.

Best Guesses

To make the most accurate range for a conception or due date, the best method is to become aware of your normal cycle. How long is your average cycle; 28 days, or 25? When do you believe you ovulate, on average? You can keep a basal temperature chart, watch cervical mucus and take an ovulation test to help determine this.

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