Confused by the estimated date of delivery

Dear doctor,
I have had several scans in my now advanced pregnancy, and each gives me a different estimated due date. Am now frustrated and confused, and unsure which date to use for my planning. Please help?

The average length of a human pregnancy is about 40 weeks. It is important to have an estimated date of delivery from the outset. This helps define certain key milestones as the pregnancy advances. You are also able to plan your working schedule, maternity leave and other aspects of your life once you have an idea about when you might expect to deliver.

As the phrase puts it, it’s an estimated date of delivery (commonly termed as EDD). It’s never an accurate date, you may deliver several days before or after the EDD. There are several ways of calculating the EDD. The date has traditionally been calculated forwards based on the first day of your last period, but this presumes regular menstrual cycles of 28 days. It is immediately obvious then that women with irregular periods may have erroneously calculated EDDs. The more precise method of calculating the EDD is based on ultrasound computations.

Pregnancy dating with ultrasound is best done in the first trimester, at around 11 to 13 weeks. Scans done at this point have the highest degree of accuracy in estimating the due date, with only a small margin of error. Subsequent scans done further into the second and third trimester have increasing margins of error, and can be erroneous by a factor of weeks. The due date should be agreed on pretty early in the pregnancy, preferably by a first trimester dating scan. The next best estimate is using the first date of your last period, if this is known accurately and on a background of regular cycles. .

So the question of varying the due date with every other scan should hardly arise. The initially agreed date should stand, regardless of what subsequent scans imply. You may however have missed the opportunity to do an early dating scan. And your periods may have been pretty irregular prior to conception. In such cases, a scan at the earliest opportunity will give a clue as to the potential delivery date. It is usually prudent to repeat the scan in a span of 3 to 4 weeks, and see whether there is conformity in estimating the due date. A big margin of error still exists though, raising caution about inducing labor too early and inadvertently contending with a premature baby.

Back to your question. Please get back to your obstetrician and review all the suggested dates backwards. You should then come up with the best estimate, which you should stick to. Any other suggested dates, especially if based on scans done in advanced pregnancy, can just be ignored.