Delivery Due Date Calculator

An early ultrasound does more than show a first glimpse of the baby: it sets the official clock for the entire pregnancy. When a scan measures gestational age directly, that measurement can date the pregnancy more reliably than period math alone. The Delivery Due Date Calculator turns ultrasound data into a delivery date. Enter the ultrasound date, the gestational age measured at that scan, and the current date, and it estimates your due date, your current gestational age, and how many days remain.

Accurate dating shapes almost every decision in antenatal care. Screening tests have narrow valid windows, growth assessments compare against expected size for the week, and the decision about when to induce an overdue pregnancy hinges on knowing exactly how far along you are. A due date anchored in ultrasound measurement is the most trustworthy version of that knowledge.

This calculator is useful whether the scan confirmed your period-based dates or corrected them. Either way, once the ultrasound has spoken, its dating becomes the reference point, and this tool carries that reference forward to today and to the finish line.

Why Ultrasound Dating Is the Gold Standard

In the first trimester, embryos grow at a remarkably uniform rate regardless of genetics, nutrition, or background. Measuring the crown-rump length, the distance from head to bottom, at 8 to 13 weeks dates a pregnancy to within about five days, far tighter than menstrual history can manage when cycles are irregular or dates are uncertain.

That precision is why clinical guidelines prefer first-trimester ultrasound dating whenever it is available and differs meaningfully from period-based dates. Later scans are less useful for dating because fetuses, like babies after birth, grow at individual rates; a large fetus at 30 weeks might simply be a big baby, not an older one. So the dating power of ultrasound is strongest early, and the calculator is designed around a scan that measured gestational age at a known date.

Once dating is established by an early scan, best practice is to keep it unchanged for the rest of the pregnancy. Chopping and changing dates from later scans creates more confusion than clarity.

How the Delivery Date Is Calculated

The logic is straightforward. A full-term pregnancy lasts 280 days from the last menstrual period equivalent. If the ultrasound showed you were already a certain number of days along on the scan date, the remaining time is 280 minus those days. Adding the remainder to the scan date gives the estimated delivery date.

Your current gestational age is the scan’s gestational age plus the days elapsed since the scan. The calculator expresses it in weeks and days, the format every antenatal chart uses. Days remaining is simply the delivery date minus the current date, giving you the live countdown.

Because the method counts forward from a measured point rather than backward from an assumed ovulation, it sidesteps the biggest uncertainties of period-based dating entirely.

The Three Inputs Explained

Ultrasound date. The calendar date the scan was performed, found on your scan report. Use the dating scan, ideally from the first trimester, rather than a later growth scan.

Gestational age at ultrasound in weeks. The age the sonographer reported, for example 12 weeks. If the report gives weeks and days, convert to a decimal: 12 weeks and 3 days is about 12.4 weeks.

Current date. The date you want the countdown calculated for. Today gives your live position; a future date lets you preview where you will be at an upcoming appointment.

Understanding Your Three Results

Estimated due date. The delivery date projected from your ultrasound dating. Treat it as the center of the normal birth window, roughly two weeks on either side, not as a scheduled appointment.

Current gestational age. How far along you are right now, in weeks and days, carried forward from the scan measurement. This is the number to quote at every appointment.

Days until due date. The countdown. Watching it shrink turns the abstract wait into something concrete, and it helps with practical planning like leave dates and nursery deadlines.

How to Use the Delivery Due Date Calculator

1. Find your scan report. Locate the dating ultrasound report with its date and reported gestational age.
2. Enter the ultrasound date. Type the scan date into the first box.
3. Enter the gestational age. Type the weeks reported at the scan, using a decimal for extra days.
4. Enter the current date. Use today or any date you want to check.
5. Press Calculate. Your due date, current gestational age, and days remaining appear instantly.
6. Use these dates consistently. Quote the same gestational age at every appointment to avoid confusion.

Worked Example 1: Scan Confirms the Period Dates

Fatima had a dating scan on 28 July 2026, which measured 12 weeks and 0 days, matching her period-based dates. She runs the calculator on 19 October 2026 to see where she stands.

The due date is the scan date plus the remaining 196 days (280 minus 84), which lands on 9 February 2027. Her current gestational age is 84 days plus the 83 days elapsed since the scan, totaling 167 days, or 23 weeks and 6 days. The countdown shows 113 days remaining.

At 23 weeks and 6 days, she is approaching viability milestones and the glucose screening window. Having the precise week count lets her book the right tests at the right time without second-guessing.

Worked Example 2: Scan Corrects Uncertain Dates

Grace was unsure of her last period date and had a scan on 10 September 2026 measuring 9 weeks and 2 days, or about 9.3 weeks. She checks the calculator on 19 October 2026.

Her due date becomes 10 September plus 280 minus 65 days, which is 215 days later: 12 April 2027. Her current gestational age is 65 days plus the 39 elapsed days, totaling 104 days, or 14 weeks and 6 days. She has 175 days to go.

Before the scan, Grace’s dates were a guess; now they are anchored in measurement. Her provider adopts the scan dating, and every subsequent appointment, from the anatomy scan to growth checks, follows the calculator’s week count. This is the scenario where ultrasound dating earns its reputation.

What the Due Date Does and Does Not Mean

The estimated delivery date is a statistical midpoint: about half of first-time mothers deliver after it, and only a small fraction deliver on it. The normal window stretches from 37 to 42 weeks, and babies arriving anywhere in that range are generally doing exactly what they should.

What the due date does determine is clinical decision-making. Before 37 weeks, providers work to delay birth; after 41 weeks, they start discussing induction, because risks slowly rise past term. Those decisions require trustworthy dating, which is why the ultrasound-anchored date matters far more than the folk idea of a baby being “late.”

If you pass your due date, try not to fixate on the calendar. Stay in close contact with your provider, attend the scheduled monitoring, and let the dating you established early guide the plan.

Milestones to Watch by Gestational Week

Around 20 weeks. The anatomy scan examines the baby’s structures in detail. It is the most comprehensive ultrasound of the pregnancy.

Around 24 weeks. Viability threshold, where intensive care becomes possible, and the window for glucose screening for gestational diabetes.

Around 28 weeks. Third trimester begins; appointments become more frequent and growth monitoring may start.

Around 36 weeks. Weekly visits typically begin, and the baby’s position is checked as birth approaches.

37 to 42 weeks. The normal birth window. Early term, full term, and late term each carry slightly different considerations your provider will explain.

Your calculator-derived week count tells you exactly when each of these arrives for you personally.

Preparing for Key Scans

The dating scan and the anatomy scan reward a little preparation. For early scans, a full bladder is sometimes requested because it improves the image; your appointment letter will say so if it applies. Bring your LMP date, cycle history, and any prior scan reports, since the sonographer uses them to interpret what they see.

The anatomy scan around 20 weeks takes longer, often 30 to 45 minutes, and examines the brain, heart, spine, kidneys, limbs, and placenta in detail. Eating normally beforehand is fine, and many clinics welcome a partner in the room. If the baby is in an awkward position, you may be asked to walk around or return another day; an incomplete scan is common and not a cause for alarm.

After each scan, store the report with your maternity notes and update the Delivery Due Date Calculator with the confirmed gestational age. Each scan refines the picture, but remember the rule: dating comes from the earliest reliable scan, while later scans monitor growth against those fixed dates.

If Baby Arrives Early or Late

About one in ten babies arrives before 37 weeks, which is classified as preterm. Late preterm birth, from 34 to 36 weeks, is the most common and usually has excellent outcomes with modern care, but any signs of labour before 37 weeks warrant an immediate call to your provider. Knowing your accurate gestational age is what lets the team respond appropriately rather than guessing.

On the other side, pregnancies continuing past 41 weeks enter the late-term zone, where providers increase surveillance with non-stress tests and fluid checks. Most will recommend induction by 42 weeks at the latest, because risks to the baby begin to rise. Your ultrasound-anchored due date is the reference that times these decisions.

Between 37 and 42 weeks, the range is simply normal. Pack your hospital bag by 36 weeks, keep your provider’s number handy, and let the countdown the calculator gives you build anticipation rather than impatience. The date was always an estimate; the baby has the final say.

Understanding Growth Scans in the Third Trimester

From around 28 weeks, some pregnancies are monitored with growth scans that estimate the baby’s weight from head, abdomen, and thigh measurements. These estimates come with a margin of error of roughly 10 to 15 percent in either direction, which surprises many parents when a “big baby” turns out average at birth.

Growth scans are compared against charts calibrated to gestational age, which is exactly why fixed, accurate dating matters so much. A baby measuring small at a correctly dated 34 weeks raises different questions than the same measurement with uncertain dates. Your calculator-derived week count is the denominator every growth assessment divides by.

If a scan suggests unusual growth, your provider may order follow-up scans two to three weeks apart to establish a trend. Single measurements fluctuate; trends tell the story. Ask what percentile the baby is tracking and whether it is consistent with earlier scans, and keep your dating fixed while growth is evaluated on its own terms.

Tips for the Countdown to Delivery

1. Use the first-trimester scan for dating. It is far more accurate for dating than any later scan.
2. Convert weeks-and-days to decimals carefully. Three days is about 0.4 weeks; small errors here shift the whole countdown.
3. Quote one consistent gestational age. Use the scan-based dates at every appointment to keep records clean.
4. Plan leave around the window, not the day. Arrange coverage from 38 weeks so an early arrival never catches you out.
5. Prepare the essentials by 36 weeks. Hospital bag, car seat, and paperwork should be ready before the birth window opens.
6. Attend all scheduled monitoring. Late-pregnancy checkups catch the issues that dating helps anticipate.
7. Track movements daily in the third trimester. Your provider will explain kick counting; it complements the calendar.
8. Discuss the post-term plan early. Ask your provider at 36 weeks what happens if you pass 41 weeks, so there are no surprises.
9. Keep the scan report accessible. Bring it to any new provider or hospital so dating never has to be reconstructed.
10. Enjoy the countdown. The days-until number shrinks reassuringly fast in the final stretch; let it build excitement, not anxiety.

Frequently Asked Questions

1. What does the Delivery Due Date Calculator do?

It projects your delivery date from an ultrasound’s gestational age measurement, and shows your current gestational age and days remaining.

2. Which ultrasound should I use?

The earliest dating scan, ideally between 8 and 13 weeks. First-trimester measurements date pregnancies most accurately.

3. Why not use a later scan for dating?

Because fetuses grow at individual rates later on. A 32-week scan measures growth, not age; using it for dating can mislead.

4. My scan disagreed with my period dates. Which is right?

In the first trimester, the scan is usually adopted if the difference exceeds a few days. Your provider makes the final call.

5. How do I enter 12 weeks and 3 days?

Convert the days to a decimal: 3 days is about 0.4 weeks, so enter 12.4.

6. Is the due date exact?

No. It is the midpoint of the normal birth window. Most babies arrive within two weeks before or after it.

7. What does 23w6d mean?

Twenty-three completed weeks and six days. Tomorrow you will be 24w0d, or 24 weeks exactly.

8. Can the due date change later?

It should not change once set from a first-trimester scan. Later scans assess growth against the established dates.

9. What happens if I go past my due date?

Your provider will increase monitoring and discuss induction, typically around 41 weeks, guided by the established dating.

10. Does this work for IVF pregnancies?

Yes, though IVF dating is already precise from the transfer date. The calculator still works if you prefer to anchor on a scan.

11. Why count in weeks and days instead of months?

Because clinical milestones are week-specific. “Six months pregnant” could mean anywhere from 24 to 27 weeks, which is too vague for care decisions.

12. My provider uses a slightly different date. Which do I follow?

Always follow your provider’s dates. The calculator is a planning tool; their records govern your care.

13. Can I check a future date?

Yes. Enter any date to see your gestational age then, useful for scheduling travel, leave, or events.

14. What is the normal birth window?

From 37 weeks to 42 weeks. Births in this range are considered term, with 39 to 40 weeks often described as full term.

15. Should I worry as the countdown nears zero?

No. The countdown reaching zero simply means you have entered the peak of the normal window. Stay in touch with your provider and wait for labor’s natural signs.

CONCLUSION

The Delivery Due Date Calculator turns your ultrasound’s precise measurement into the three numbers that organise the rest of your pregnancy: the estimated delivery date, your current gestational age, and the days remaining. Anchored in first-trimester science rather than calendar guesswork, these dates let you schedule every test at the right moment and plan the practical side of birth with confidence.

Enter your scan details, learn your week, and keep those dates consistent at every appointment. The countdown is on, and now you know exactly where you stand in it.