Establishing a ‘Due Date’

The pregnancy confirmation visit is a joyous time to celebrate the ‘official’ recognition of a pregnancy.  During this visit questions are asked about the first day of last menstrual period (LMP), how regular the periods are, any bleeding since the last period, and how you are feeling.  However, even with the best account of periods there is rarely the identification as to when conception occurs.  Because sperm are viable for up to 5 days it gives some variability about when conception occurs.  Ovulation kit use can help but there is variability around the strength of the measurement of the hormone found in the urine. Accuracy of dating is important to prevent unnecessary intervention on either side of the ‘term pregnancy’ definition of 37-42 weeks gestation.

So what is the best way to determine the due date?  With early entry into care (< 10 weeks) the due date is more accurate when the size of the growing uterus, determined by a pelvic exam, correlates with the LMP.  Ultrasound in the first trimester improves the accuracy of final EDD.  Early ultrasound prior to 10 weeks of pregnancy is accurate within 5 days.  So if an early ultrasound determines that the fetus is smaller or larger by 5 days of the LMP then the EDD can and should be changed to the ultrasound finding.

So why are we concerned about establishing an accurate EDD?  Evidence shows that babies born at term (37-42 weeks) have the best outcomes and the ultimate goal of care for pregnancy and birth is having the best outcome! As a provider I have heard over the years of women that had babies at 42-44 weeks without any problems. These women generally did not have good data to confirm the pregnancy early and no early ultrasound leaving variability to the accuracy of the EDD.  It can be very frustrating to come to the end of your prenatal care, only to have a discussion about whether the EDD that we have all been using may not be accurate.

It is especially frustrating for the provider and client planning an out of hospital birth and the birth time falls either before or after the 37-42 week definition for the pregnancy. So establishing the due date based on the best information is critical to prevent unnecessary intervention such as induction of labor. As midwives we strive to clarify the due date so we can create the best plan with our families for an low intervention pregnancy and birth. There will be a blog post discussing postdates pregnancy posted later as a part of this series.