How to Accurately Predict Your Baby’s Arrival
Wondering exactly when your little one will make their grand entrance? You’re not alone. Whether you’re tracking every twinge or just trying to plan ahead, figuring out your baby’s due date is one of the first big questions of pregnancy. But how accurate is that circle on the calendar—and how do doctors even come up with it?
There’s more than one way to predict when your baby might arrive, and some are more precise than others. From counting weeks from your last period to using IVF dates or early ultrasounds, each method has its quirks. In this guide, we’ll break down the most common tools used to estimate due dates and help you understand what’s most likely—and what’s just an educated guess.
Ways to Figure Out Your Baby’s Arrival Date
Counting from the Start of Your Last Period
Most doctors start with the first day of your last menstrual period (LMP). Then they add 40 weeks, or about 280 days, from that date. This approach is common because many people remember when their last period began.
Pregnancy is called 40 weeks instead of 38 because ovulation and conception usually happen about two weeks after your cycle starts. This method includes both the time before and after fertilization. Even if your cycle is not the standard 28 days, healthcare providers usually begin with this method.
Calculating Based on When You Got Pregnant
If you know the date you conceived—maybe from ovulation tests or tracking cycles—you can use another method. Count 38 weeks, or 266 days, from the day conception happened.
Knowing the exact conception date can be tricky because sperm can survive up to five days, and ovulation may occur a few days after intercourse. Still, if you are confident about the date, this method gives a direct estimate.
Using the IVF Embryo Transfer Day
If you became pregnant through in vitro fertilization (IVF), use the embryo transfer date to estimate your due date. For a Day 5 embryo transfer, add 261 days to the transfer date. For a Day 3 embryo transfer, add 263 days. This method gives a reliable estimated due date for IVF pregnancies.
First Pregnancy Ultrasound Check
Doctors often do an ultrasound scan early in pregnancy to estimate the baby’s due date. This test is sometimes called a dating scan. Doctors and midwives use results from this first ultrasound to see how far along the pregnancy is.
The timing and use of a first ultrasound can vary:
- Some providers give an early scan to all patients.
- Others only recommend it if it is hard to tell the due date in other ways.
A first ultrasound may also be suggested if you have risk factors, such as:
- Previous pregnancy problems
- A history of miscarriage or pregnancy loss
- Irregular menstrual cycles
- Difficulty getting pregnant
- Certain ongoing health issues
- Age 35 or over
The ultrasound scan uses sound waves to create pictures of the baby and uterus. This helps check the baby’s growth and confirms the expected delivery date. In many cases, this method is more accurate than using the last period date or guessing the conception date.
Can Your Due Date Be Revised?
A due date, or estimated date of delivery (EDD), can change during pregnancy. If an ultrasound in the first trimester shows that the baby is much larger or smaller than expected, the provider may adjust the due date.
This happens often if the menstrual cycle is not regular, making it hard to know exactly when conception happened.
How Accurate Is the Estimated Due Date?
Most women do not deliver on their exact due date. Only about 5% of pregnancies reach childbirth on that predicted day. Labor is just as likely to start in the two weeks before or after the due date.
- Full-term birth happens between 37 and 42 weeks of pregnancy.
- Babies born before week 37 are considered preterm.
When to Take a Pregnancy Test
The best time to take a pregnancy test is a few days after a missed period. This timing gives a more reliable result because the pregnancy hormone, hCG, increases after ovulation and fertilization.
Most home pregnancy tests check for hCG in urine. Testing too early can lead to an inaccurate result, even if the test claims it works before your period is due.
Blood tests at a healthcare provider’s office are more sensitive. These tests can sometimes detect pregnancy six to eight days after ovulation. Cycle length and ovulation timing can affect when a test shows an accurate result.
What Moms Say About Due Date Accuracy
Main Findings
Mothers have mixed experiences with due date accuracy. Only a small portion actually give birth on the predicted day. Among several mothers, stories about late and early arrivals are common:
| Mother | Baby’s Arrival |
|---|---|
| Bremgoe91 | Two arrived two weeks late, third came a week early |
| 8Daisy | Four children, all born on or near their due date |
| GothHoney87 | Both children arrived at 41 weeks and 2 days |
| Hannahkovachick | First baby came at 38 weeks, 5 days |
| Ninalynn05 | One on due date, second a week late |
Most babies do not match the exact estimated due date given by healthcare providers. Research shows that only about 5% of deliveries happen on the scheduled due date. Many mothers go into labor during the two weeks before or after this date. The due date is a helpful guide for planning and tracking a baby’s growth, but early and late arrivals are both normal.
