How to calculate how many weeks pregnant you are (it’s trickier than it sounds)
When thinking about how many weeks pregnant you are, it’s easy to get confused.
But knowing exactly how far along you are in your pregnancy is vital to receiving accurate prenatal care.
The length of your pregnancy depends on a number of factors. It can be more complicated than you’d think.
Some people measure in weeks, and some in months. Everyone wants to know your due date, but how exactly is that date determined?
The reason tracking how many weeks pregnant you are is so confusing is because of a commonly held myth. Everyone things of a pregnancy as lasting 9 months.
That’s not necessarily true.
The method of dating a pregnancy by months is not used by your health professional.
Instead, they use a more accurate method. This method is based on the number of weeks that have passed since you became pregnant.
Ultimately, your due date is measured by the date of your last period.
How Long Is Pregnancy?
On average, a pregnancy lasts 40 weeks or 280 days from the first day of your last menstrual period.
The number of weeks you are pregnant backs out to the date of your last period.
Your doctor calculates pregnancy length by the date that you ovulate.
Your due date is at week 40. Using the date of ovulation, pregnancy is 266 days long on average.
How Many Weeks Pregnant Am I?
Use a calendar to calculate how many weeks pregnant you are.
List the first day of your last period. Then count forward from your last period to determine which week is when. It will be the same day of the week every week.
First day of last period: January 1
Week 1: January 8
Week 2: January 15
Week 20: May 21
Due Date: October 8
If you know your due date but not when you got pregnant, simply reverse the process to find out when you became pregnant.
There are online calculators as well.
Ask your doctor at your next appointment if you are still unsure how far along you are.
Why Weeks Instead of Months?
A month is too long when it comes to calculating how many weeks pregnant you are.
Calculating a pregnancy by weeks is easier and more accurate for doctors.
Each day of pregnancy is an important one. So it is imperative that doctors have a good idea of how far along you should be.
Babies grow quickly. Just one week can make a huge difference in their development.
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The weekly calendar structure allows your doctor to check that your test results are falling in line with averages. It also helps identify any issues with the pregnancy and helps schedule care between your entire medical team.
Alternative Ways to Date Your Pregnancy
There are times when using the first day of your last normal period is not the best method when assessing how far along you are.
It’s possible that you do not know the date of your last period. You might have high variance in your cycles.
One alternate way to determine how many weeks pregnant you are is an ultrasound examination in the first trimester.
Not every woman has an ultrasound one in the first trimester. Typically, “high risk” pregnancies have ultrasounds in the first trimester.
During the ultrasound, your baby’s length in utero is compared to growth milestones to determine how many weeks he’s been growing. Used together with your period dates, this can be an accurate way to assess pregnancy length.
Additionally, you can ask your doctor if they have a gestational wheel calculator. These give you a visual representation of your last period and due date.
There are apps on your phone that will fulfill the same function, but some are better than others.
If your pregnancy is the result of in vitro fertilization or another assisted reproductive technology, talk to your reproductive endocrinologist. They will be able to determine your due date based on the age of the embryo and the date of transfer.
Adjustments to Your Due Date
The American College of Obstetricians and Gynecologists (ACOG) states that any pregnancy that does not have an accurate due date before 22 weeks gestation is suboptimal.
Due date adjustments are common. As many as 26 percent of women will have an adjustment made to their due date.
Make sure practitioners give a thorough and detailed explanation if they change your due date. The due date should not change drastically.