Doctors typically estimate a woman’s due date based on the first day of her last menstrual period (LMP). This is because most women ovulate about 14 days after the start of their period. Fertilization usually happens shortly after ovulation. Since a full-term pregnancy lasts around 40 weeks, doctors can count 40 weeks from the start of the woman’s LMP to estimate the due date.
How accurate are due date estimates?
Due dates estimated from a woman’s LMP are accurate within about 1-2 weeks for about 90% of pregnancies. However, some factors can make the due date less accurate:
- Irregular menstrual cycles
- Birth control use near conception
- Bleeding early in pregnancy
- Being uncertain of the date of the LMP
If the LMP date is uncertain, doctors may use an ultrasound to get a better estimate of the due date. Ultrasounds done early in pregnancy are more accurate for dating a pregnancy than ultrasounds done later on.
How do ultrasounds help estimate the due date?
During an ultrasound, the doctor measures the size of the fetus to estimate the gestational age and due date. Early in pregnancy, the fetus grows at a predictable rate, so the size gives a good indication of how far along the pregnancy is. Here’s how ultrasound dating works:
- Crown-rump length: Measured between 6-12 weeks. Accuracy about 5-7 days.
- Head circumference: Measured between 12-24 weeks. Accuracy about 7-10 days.
- Femur length: Measured after 14 weeks. Accuracy about 10-14 days.
- Head/abdomen/femur: Measured between 18-24 weeks. Accuracy about 14 days.
Later ultrasounds are less reliable for dating because growth starts varying more between different pregnancies around 24 weeks.
What if my due date seems wrong?
Sometimes a woman knows the exact date of conception and her estimated due date doesn’t match up. This can happen for several reasons:
- You ovulated earlier or later than expected in your cycle
- You conceived shortly before or after the estimated conception date
- There was an error in LMP dating if your cycles are irregular
If your due date seems more than a couple weeks off, talk to your doctor. They may want to do an ultrasound to get a better due date estimate. However, keep in mind that even ultrasound dating is not 100% accurate. The main thing is that your baby keeps growing and developing normally throughout pregnancy, not the exact due date.
How accurate is a due date?
While doctors provide an estimated due date, only about 5% of women actually give birth on their exact due date. Most babies come within a window of 2 weeks before or after the due date. Here is how likely delivery is for various weeks around the estimated due date:
| Weeks from due date | Likelihood of delivery |
|---|---|
| 3 weeks before due date | Less than 1% |
| 2 weeks before due date | 3-5% |
| 1 week before due date | 15-20% |
| Due date | 50-71% |
| 1 week after due date | 15-25% |
| 2 weeks after due date | 5-10% |
| 3+ weeks after due date | Less than 5% |
As you can see, only a small percentage of women actually deliver right on their due date. Doctors consider a pregnancy full-term anytime between 37-42 weeks. Most women go into labor naturally during this window.
When will doctors induce labor?
Many doctors will let women go up to 2 weeks past their due date before inducing labor. This gives time for natural labor to start on its own. However, some medical conditions make it unsafe to wait that long past the due date. Doctors typically recommend labor induction if:
- You are more than 1-2 weeks past your due date
- Tests show the placenta is no longer working well
- There are signs the baby is not doing well in the womb
- You have a medical condition like diabetes or high blood pressure
- Your water breaks but labor does not start naturally
Labor induction helps lower risks when the pregnancy goes too far past the estimated due date. However, induction does increase the chances of needing interventions like C-sections. Many doctors will discuss the risks and benefits of waiting versus inducing labor if you go past your due date.
Does the baby’s gender impact the due date?
Some research shows that a baby’s gender could affect when labor starts. However, the difference is quite small. On average:
- Male fetuses tend to go about 1 day longer than estimated due dates
- Female fetuses tend to go about 2 days shorter than estimated due dates
However, this is only an average. Individual pregnancies can vary widely for many reasons. Doctors do not change due date estimates based on the baby’s gender. The effect is too small to be useful.
Can stress or environment influence the due date?
High levels of stress have been associated with an increased likelihood of preterm birth (before 37 weeks). However, for full-term births, maternal stress and anxiety do not appear to influence the timing of labor onset around the estimated due date. Environmental factors like barometric pressure and lunar cycles were once thought to impact due dates, but research does not show strong evidence for these effects.
How can I estimate my due date before seeing a doctor?
If you know the first day of your last period, you can get a rough estimate of your due date before seeing your doctor. Here is how:
- Mark the first day of your last period on a calendar.
- Count forward 40 weeks from that date.
- This estimation gives you a due date that is 280 days from the start of your last period.
Online due date calculators can also help estimate your due date and conception window if you know the date of your last period. However, doctors will confirm at your first prenatal visit using additional information like ultrasound results.
When to see a doctor
See your doctor as soon as you think you may be pregnant. They will help confirm the pregnancy using tests and help establish an accurate due date. Prenatal care early in pregnancy is important for mom and baby. Let your doctor know right away if your estimated due date seems way off or does not match up with ultrasound measurements later on.
The bottom line
Doctors estimate your due date based on the first day of your last period. They will adjust the due date if needed as pregnancy progresses. However, the due date is just an estimate – very few women actually deliver on their exact due date. Labor onset varies naturally by a couple weeks in most cases. While doctors provide an estimated due date, the main priorities are that mom and baby stay healthy throughout pregnancy and that labor starts on its own when ready.
