Understanding the “Gestational Age” of your pregnancy

The number of weeks in our ultrasound report seems off. The report says Sep 14 was Week 24. However, based on the estimated due date (Dec 20th Week 40th), the 28th week started on September 27. Can you explain this discrepancy? I am confused.
–Brianna & Paul

Dear Brianna & Paul…

If you’re planning your travel to pick up your baby, you may be tempted to buy your airline tickets to arrive on the Estimated Due Date (EDD). But for most of your pregnancy the EDD in the report is way off. That’s because the “age” of the pregnancy isn’t actually the number of weeks that you’ve been pregnant. The EDD is based on the “Gestational Age” of the fetus, not the “Chronological Age”.

Gestational Age is quite different than the chronological age, and can vary a lot (especially early in the pregnancy). Gestational Age is actually a measure of the fetus’ development relative to a ‘typical’ pregnancy. So when a report shows the age at 14, that doesn’t mean that the pregnancy has been ongoing for 14 weeks — it means the fetus is developed at the level of a typical 24 week fetus. Gestational Age can fluctuate significantly week by week — some weeks the fetus has a growth spurt, and gestational age jumps forward. Other weeks the growth is slow, and the gestational age slows down as well.

As you get closer to the delivery date, the gestational age begins to more closely coincide with the chronological age. Likewise, the estimated due date becomes more reliable. By the time you are midway through the third trimester, your due date can be pretty accurately predicted. But making travel plans earlier in the pregnancy can be risky (unless you by fully changeable airline tickets).

If you base the due date on the gestational age, then you can see how the planned arrival can move around a lot early in the pregnancy. Small changes in gestational age are magnified by the long time before the delivery. A minor delay in Gestational Age early in the 2nd trimester can drastically change the final due date, but the same delay in the 3rd trimester won’t be as significant.

Also the delivery date is likely not 40 weeks (either chronologically or by Gestational Age). A baby is considered fully formed at 38 weeks gestational age — which is quite often as early as 37 weeks chronologically. So the due date may be earlier than you anticipate. Plus at this early stage, the due date is going to fluctuate because the gestational age also fluctuates. The earlier you are in the pregnancy, the more significant the fluctuation will effect the final delivery date.

I hope this makes sense. IMO, buying airline tickets in the 2nd trimester is quite risky. The due date is likely going to change. You should make sure that all travel arrangements can be changed with a minimal financial penalty.

I hope this is helpful,
— Bill