How Do They Determine Due Date? | What The Scan Is Saying

A due date is a shared calendar target, not a stopwatch. It helps time prenatal visits, schedule screening windows, and plan work leave and logistics. It also gives your care team one “week count” so everyone is talking about the same point in pregnancy.

That week count has a name: gestational age. It is counted from the first day of the last menstrual period, and conception usually happens later. This convention keeps dating consistent across clinics and labs.

What A due date means in practice

Your estimated due date (often written as EDD) marks 40 weeks of gestational age. It’s the anchor used for prenatal timing, growth checks, and decisions near the end of pregnancy.

Ovulation and implantation can vary from person to person. So the earliest reliable data matters more than a guessed conception day. Many offices confirm dating with an early ultrasound, then keep one “best obstetric estimate” in the chart.

How Do They Determine Due Date? Steps clinicians use

Most clinics follow a simple sequence. They start with your cycle history, then add ultrasound data when it’s available. Next, they decide which date will be used in the record from then on.

Step 1: Start with the first day of the last menstrual period

If you know the first day of your last period, the classic calculation adds 280 days (40 weeks). Clinics often begin here because it’s fast and it gives an early working estimate.

If your cycle is longer or shorter than 28 days, your ovulation day may shift. Some clinicians adjust the count by the difference between your usual cycle length and 28 days, then check whether early ultrasound lines up.

Step 2: Check dating against an early ultrasound

An ultrasound in early pregnancy measures the embryo or fetus and converts that size into gestational age. In the first trimester, crown–rump length is commonly used. Many systems offer a dating scan in the 10–14 week range. NHS guidance on the 12-week scan notes that a dating scan gives a more reliable due date.

Professional guidance also explains how last period dating and ultrasound dating are combined, plus when a date change makes sense. ACOG’s methods for estimating the due date lays out that approach.

Step 3: Choose one date and keep it steady

Once your due date is set, most practices avoid changing it again. Small differences on later scans can come from measurement angle, baby position, or normal growth variation. Switching the date late can throw off test timing and growth tracking.

Which dating method tends to win when dates clash

Sometimes the last period date and the first ultrasound don’t line up. When that happens, clinicians weigh the reliability of each input.

When the last period date carries more weight

A last period date carries more weight when cycles are regular and you feel confident about the start day. People who track bleeding patterns often have cleaner dates, which can match early ultrasound closely.

When ultrasound tends to take priority

Ultrasound often takes priority when cycles are irregular, bleeding was unusual, you stopped hormonal birth control recently, or the period start date is unclear. Early ultrasound can also be preferred when there’s a large gap between dates.

When conception timing is known

Some pregnancies come with known timing. IVF is the clearest case, since embryo transfer dates are recorded. In that setting, the clinic calculates gestational age from procedure dates, then matches ultrasound growth to that timeline.

What clinicians measure on ultrasound for dating

Different measurements are used at different points in pregnancy. Early on, embryo length is the main data point. Later, the head, abdomen, and femur length contribute to gestational age estimates.

MedlinePlus explains that gestational age can be estimated using ultrasound measures of the baby’s head, abdomen, and thigh bone. MedlinePlus on gestational age gives an overview of this measurement approach.

First trimester: Crown–rump length

Crown–rump length (CRL) measures from the top of the embryo’s head to the bottom of the torso. It’s widely used for dating because early growth patterns are similar across many pregnancies.

Second trimester: Head, abdomen, and femur

As pregnancy advances, the fetus curls and stretches more, so CRL becomes less useful. Dating then relies on biometric measures such as head size and femur length. The error range is wider than it is early on.

Third trimester: Dating gets less precise

Later in pregnancy, babies vary more in size. That makes late dating less precise, so clinicians prefer to keep the earlier due date when they have it. When you hear “measuring ahead,” it often means size compared with the current due date, not a new due date.

Dating rules and the calendar math

Clinics use guidelines so dating is consistent across providers. One common anchor is Naegele’s rule, which converts the last period date into an estimated due date. Johns Hopkins shows the calendar steps and notes the method assumes a 28-day cycle. Johns Hopkins on calculating a due date walks through the calculation.

Modern practice also uses clear thresholds for when an ultrasound difference is large enough to change the date, especially early in pregnancy. The core idea is simple: earlier scans are tighter for dating, later scans are better for growth.

Dating inputs and how they shape your estimate

Think of due date determination as a puzzle with a few pieces. Some pieces set the date. Others explain why an early estimate might shift after the first scan.

• Last menstrual period date: Sets the starting point for the initial estimate and the week count.
• Cycle length and regularity: Helps interpret how well the last period date reflects ovulation timing.
• Early ultrasound: Often confirms or corrects the estimate based on fetal size.
• Known conception or IVF dates: Adds a fixed timeline that can be matched to ultrasound growth.
• Multiple pregnancy: Changes visit schedules and growth patterns, yet dating still follows standard rules.

Dating method comparison table

The table below shows the main inputs clinicians use, when they’re most useful, and what tends to make them drift.

Dating input	When it works best	Common reasons it drifts
Last menstrual period + 280 days	Regular cycles and clear period start date	Cycle length varies; ovulation timing shifts; spotting confused as a period
Adjusted last period method	You know your usual cycle length	Ovulation still varies; cycle history is an average, not a guarantee
First-trimester ultrasound (CRL)	Early scan with a clear measurement	Late first scan; measurement limits from fetal position
Second-trimester biometry	No earlier scan is available	Natural size variation grows; measurement error widens
Third-trimester biometry	Dating is unknown and care starts late	Wide normal size spread; growth issues can mimic wrong dates
IVF / embryo transfer dating	Procedure dates are recorded	Chart entry error in embryo age or transfer day (rare)
Known ovulation (LH tracking)	Consistent tracking with a clear surge pattern	Surge-to-ovulation timing varies; implantation timing varies
Early ultrasound plus sure last period date	Both sources line up closely	Small differences are normal and do not mean a new due date

What to do if your due date changes

A revised date can feel unsettling. Most of the time it’s just your clinic switching from an early estimate to a better one.

Ask what data triggered the change

Was the change based on an ultrasound measurement, or on a corrected last period date? If it was an early scan, the new date often becomes the record’s anchor.

Check which date is used for scheduling tests

Many screenings and anatomy scans are timed by gestational week. Once your clinic updates the date, those appointments should be interpreted against the new week count.

Keep one date in your own notes

Use the date your clinic calls the “official” due date in your calendar, then track pregnancy weeks from that point. This reduces mix-ups when booking visits or filling out forms.

Questions that clear confusion fast

These questions keep the discussion focused on the data your team used.

• Which date is used in my chart? Ask whether it’s based on the first ultrasound or the last period date.
• What measurement was used on my first scan? Early scans often use CRL; later scans use head, abdomen, and femur measures.
• Do you expect my due date to stay fixed now? Most clinics prefer one steady date after early dating is complete.

Milestones tied to gestational weeks table

This table shows common timing anchors that many clinics schedule by week count. Your clinic may use a different schedule.

Gestational week range	What is often scheduled	Why the timing matters
10–14 weeks	Dating scan and early screening options	Earlier measurements are tighter for dating; screening windows are time-based
18–22 weeks	Anatomy scan	Structures are easier to see while there is still room to move
24–28 weeks	Glucose screening in many practices	Timing aligns with common screening windows
35–37 weeks	Group B strep screening in many practices	Testing close to birth helps guide labor antibiotics if needed
40–41+ weeks	Extra monitoring or induction planning in some cases	Week count guides decisions once pregnancy passes the due date

One steady due date keeps your care calendar consistent. The goal is the earliest reliable dating data, then a fixed reference point for the rest of pregnancy.