How your due date is calculated
The “due date” is an important reference date for scheduling tests and appointments, but it is not very helpful in figuring out when the baby is coming. Most women will deliver within one week of the due date. Plenty of women deliver after their due date too. So, when talking about labor, the due date is not so helpful. Put simply, your due date is at 40 weeks.
The Due Zone
The time frame between 35 and 42 weeks is what we call your due zone. Babies are essentially ON TIME if born in the due zone. The due date is based on the first day of your last period and may be confirmed by or changed by the FIRST/earliest ultrasound only. (“First ultrasound” means: the first time we see the fetal heart beat on ultrasound.)
Once the due date is established, IT WILL NOT CHANGE. Every time we do pregnancy ultrasound the MACHINE spits out a due date, based on the average measurements, on that day. Instead of changing the due date, we can assess baby’s growth. More accurate than a new due date, the later ultrasound should give a birth weight percentile, based on the known due date. Normal growth is between the 10th and 90th percentile.
How is a due date set?
The due date is calculated by Naegele’s Rule. Take the first day of the last period: add seven days to the date and count back three months. For example, if the period was June 15th then the due date is March 22nd.
First day of the last period – ___ / ___ / ______.
Due date based on your period – ___ / ___ / ______.
The FIRST ultrasound showed the pregnancy at _____ weeks and _____ days, based on fetal size.
On the date of the first ultrasound, the gestational age based on the period, is _____ weeks and _____ days
YOUR DUE DATE IS : ___ / ___ / ______ , based on the comparison of the period and ultrasound.
Your due date will not change
Here are some helpful dates to know:
- 22 to 24 weeks: Viability. Babies born much before 22-24 weeks typically can not live on the outside. A delivery much before 22-24 weeks is considered a miscarriage. Right around 22-24 weeks is the beginning of “prematurity.”
- 35 weeks: The finish Line. There is no medical indication to stop labor from coming after 35 weeks. In fact, some research suggests that labor starting after 34 weeks should not be stopped.
- 35 to 42 weeks: The Due Zone. This five week window of time, with the due date somewhere in the middle, is considered on time. Babies are not considered early or late if they are born in the due zone.
- 40 weeks: The Due Date.
- 42 weeks – The End Date. Most pregnancies should be delivered by 42 weeks, or TWO weeks past the due date. Research shows that babies begin to have problems in pregnancy, just because of being inside too long. So, the recommendation is that labor is induced by 42 weeks.
What about 9 MONTHS?
Months can be figured out based on your due date. The Due Date is at nine months. So if you count backwards… if the due date is January 1st, then 8 months is December 1st, 7 months is November 1st, and so on. MONTHS and WEEKS do not match up: don’t try to divide by four to convert from weeks to months.