There are several different ways to calculate your due date, including:
Calculate due date
Your due date is considered 40 weeks (280 days) after the first day of your last period or 38 weeks (266 days) after ovulation. Even if an early ultrasound is used to determine or change your due date, it is still based on the basic idea of a 280-day gestation period.
Assuming you know the date of your last period, your primary care provider will use this information to figure out the due date. Many midwives and obstetricians use the pregnancy wheel, a simple device that quickly provides 280 days from your last period.
There is a formula called Naegele’s rule. This is what is used for the due date calculator at the top of this page. That’s also how the wheel of pregnancy works. You can calculate the date yourself using Naegele’s rule.
What is Naegele’s rule?
To find out your due date based on your menstrual cycle:
- Record the date of your last period
- One more year
- Seven more days
- Move the date back three months
For example, if the first day of your last period is August 11, 2021, you want:
- 1 more year (created on August 11, 2022)
- 7 more days (becomes August 18, 2022)
- Move date back 3 months (until May 18, 2022)
- Your date due will be May 18, 2020.
If your fertility app gives you a different date than the one calculated by your doctor, it’s most likely based on your ovulation date, not the date of your last period. If you ovulate earlier or later than day 14 of your cycle, this will change your due date respectively.
Ovulation versus menstruation
Your due date in relation to ovulation is considered more accurate than your due date calculated based on your last period. If you know when you ovulate, or you know your cycle is longer than average, share that information with your doctor.
The dates may be a week apart, but that week can make a big difference. For example, if you have a C-section scheduled, you don’t want to schedule it too early. If your doctor is trying to decide if your pregnancy is past your due date, you can wait an extra week before considering induction.
An ultrasound during the first trimester of pregnancy — before 13 weeks — can be used to provide a due date or confirm a due date determined by your last period. Not every mom-to-be needs one unless the date is uncertain.
Transvaginal ultrasound is usually done between 9 weeks and 13 weeks of gestation. During the procedure, the technician measures the length of the fetus from head to rump. This is how they estimate how old the baby is, and from there they come up with an estimated due date.
Another method, called biparietal diameter (BPD) ultrasound, measures the diameter of the baby’s skull and may even be more accurate than other methods.
With all that said, the ultrasound’s due date is still far from perfect. If the due date for the ultrasound is different from the date determined by your last period, both of these dates should be recorded in your medical record.
If your ultrasound due date is less than seven days different from your menstrual cycle due date, your due date will not change. If it is more than seven days past due, your due date may be changed.
Use IVF due date
If you have had IVF treatment, your due date will never change. The IVF due date is not determined by your last period, the date of conception or even the date of egg collection. They are determined by the date of embryo transfer and the age of the embryo at the time of embryo transfer.
With IVF, the due date is determined by the date the embryo implants in the uterus. Because the transfer date is a fixed date from the time of the initial blood draw, doctors can calculate more accurately.
Role of Base Height
Your midwife or doctor may measure your base height during your pregnancy checkup. Basal height is the measurement in centimeters from your pubic bone to the top of your uterus. It will grow at a predictable rate as you continue to get pregnant.
After 20 weeks, your base height in centimeters will usually equal the number of weeks you were pregnant. In other words, at 21 weeks pregnant, your base height should be around 21 centimeters (10 inches).
Sometimes the base height doesn’t match completely. Slight variations are normal, but if you are measuring much smaller or larger than expected, your primary care provider may want to investigate with another ultrasound.
While useful, baseline height is not an accurate gestational age measurement and will not affect the estimated due date in any way.
Why is your due date important?
Your due date is probably one of the first pieces of information you’ll look for after finding out you’re pregnant. Friends and family will want to know so they can look forward to welcoming your new baby (and supporting you).
Doctors, midwives, and nurses will want to know so they can monitor their health milestones and make decisions about prenatal testing and interventions.
Your due date (sometimes called the expected delivery date) is less than a deadline and more like a timeline. Your due date indicates either week 40 from your last period or week 38 from ovulation.
Once your due date is calculated, you will likely give birth within the four-week period surrounding your due date. In other words, sometime in the two weeks before and two weeks after the due date is set, you may be able to see your baby.
About 70 out of 100 pregnant women will give birth within 10 days of their due date. Assuming there are no complications, others will give birth a little earlier or a little later.
Due dates are valid for many reasons, including preparing your home and family for the new arrival. There are a number of other reasons as well, it’s helpful to know your due date.
Monitor fetal development
Your healthcare provider will consider what week of pregnancy you are in when determining if your pregnancy and fetal development are on track. Your doctor will look for several indicators, such as:
- When will a heartbeat show up on a transvaginal ultrasound?
- When can a heart tone be detected with a handheld Doppler?
- When should babies start moving?
- What is the base height?
Schedule a test
Some prenatal testing needs to be done within a specific time frame. For example, ultrasound to determine viability should not be performed before six weeks.
The AFP blood test — used to screen for birth defects — is ideally done between 16 and 18 weeks. An ultrasound to determine if the twins are sharing a placenta or amniotic sac should be done. from week 11 to week 14 of pregnancy.
Determine if preterm labor or not
Babies born too early are at risk of many health problems. There are steps we can take to slow or pause preterm labor. With that said, there are also risks to the drugs used to stop preterm labor, for both mother and baby.
You and your doctor must decide together when the risks of allowing labor to continue outweigh the risks of intervention. This is partly determined by knowing what week you are in.
There are also risks to the baby and mother if the pregnancy lasts too long. Postpartum pregnancy is unborn at 42 weeks (14 days after the due date). If this happens, your midwife or doctor may decide to induce labor.
Average length of pregnancy
While 40 weeks is the average, it’s not unusual to have your baby earlier or later. Previously, 37 weeks was considered a full-term pregnancy. This definition has two implications for when it is safe to induce labor or schedule a cesarean section.
However, we now know that delivering at 37 weeks can increase health risks. It’s not nearly as dangerous as giving birth before 37 weeks, but it’s not ideal either.
In response, the American College of Obstetricians and Gynecologists (ACOG) created new definitions to describe the timing of labor:
ACOG Definition of Pregnancy
- Beginning: 37 weeks to 38 weeks and six days
- Full term: 39 weeks to 40 weeks and six days
- End of term: 41 weeks to 41 weeks and six days
- Post-production: any pregnancy beyond 42 weeks
Sometimes, when talking about twins or triplets, you’ll hear people say things like “Full term twins at 37 weeks”. But this is not really correct. All pregnancies – including twins, triplets, or more – don’t really reach full term until they’re 39 weeks.
While you may have a higher risk of early labor with twins, that shouldn’t affect how labor time is classified.
Factors affecting the due date
With so much focus and excitement in an assigned day, you’d think that would be an accurate number – but that’s not always the case. The simple truth is that only 5 out of 100 women will give birth on their actual due date.
However, determining what week of pregnancy you are in is important for getting good prenatal care and planning for your upcoming life change.
The majority of babies are born within a 35-day period, with births occurring between about 37.5 weeks and 42.5 weeks. The due date also has no deadline. You can give birth a week later than your due date and still be considered on time.
There are many factors that can affect your due date. First, every pregnant woman has her own predetermined schedule. Many people will tell you that their pregnancies are all about the same length. If you’ve had two children, each born at about 41 weeks, your third child is likely to arrive around 41 weeks.
First-timers tend to go into labor later. You will most likely give birth to your first child a few days before your due date.
And finally, some people are at risk for preterm labor. About 1 in 10 pregnancies end in preterm birth. Maternal risk factors for premature babies include previous preterm birth, vaginal bleeding during pregnancy, diabetes, high blood pressure, smoking, and even periodontal disease.
A very good word
Your due date helps you and your healthcare providers track your pregnancy and plan for labor and delivery. It’s an important date, but it’s not a deadline. You can give birth before or after your due date — and it might be fine.
If your due date comes and goes, you may start to wonder if you’ll be pregnant forever. Do not worried. While those final days can feel like endless, your baby will arrive when he’s ready, not a minute earlier.
To make sure the baby isn’t born too early, ACOG recommends that labor induction not be considered until the start of week 41 (or seven days after your official due date).
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