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Pregnancy Dating Explained: Last Menstrual Period, Ultrasound and Due-Date Changes

How Pregnancy Due Dates Are Calculated

In This Article

A pregnancy due date starts as an estimate based on the first day of your last menstrual period (LMP), then gets checked — and sometimes adjusted — using an early ultrasound. Both methods estimate the same thing, gestational age, but they don’t always agree, and that’s normal. This page explains how each method works, why your provider might change your due date, and when timing is worth flagging early in a pregnancy conversation.

Gestational Age vs. Fetal Age: What the Terms Mean

Gestational age is the standard way pregnancy is measured. It counts weeks from the first day of your last menstrual period, not from conception. Because ovulation and fertilization typically happen about two weeks after that first day, gestational age generally runs about two weeks ahead of the baby’s actual biological age. A full-term pregnancy is usually described as 38 to 42 weeks gestational age. This is a naming convention used across prenatal care, not a judgment about any individual pregnancy.

How LMP Dating Works

By convention, a due date is set at 280 days after the first day of your last period. This method assumes a regular 28-day cycle with ovulation around day 14, then counts forward 40 weeks. It’s simple and doesn’t require any equipment, which is why it’s often the first estimate given. But it depends on two things being accurate: knowing exactly when your last period started, and having a cycle that’s close to the 28-day average. If your cycles run long, short, or irregular, or if the start date isn’t clear, LMP dating alone can be off.

How Ultrasound Dating Works

An early ultrasound measures the embryo or fetus directly instead of relying on cycle assumptions. In the first trimester, up to about 13 weeks 6 days, this is usually done by measuring the crown-rump length (CRL) — the length of the embryo from head to bottom. According to the American College of Obstetricians and Gynecologists (ACOG), first-trimester CRL measurement is the most accurate way to establish or confirm gestational age, with an accuracy of about plus or minus 5 to 7 days. That accuracy is highest earlier in the first trimester and gets less precise as pregnancy progresses, since babies grow at different rates later on. By the third trimester, ultrasound-based dating is the least reliable, with an accuracy of roughly plus or minus 21 to 30 days.

LMP Dating vs. Ultrasound Dating: A Quick Comparison

  • Last menstrual period (LMP) — What it uses: the first day of your last period. Accuracy: good if your cycle is regular and you’re certain of the date; less reliable with irregular cycles or an uncertain start date. When it’s used: as the first estimate, often before any ultrasound is done.
  • First-trimester ultrasound (CRL) — What it uses: a direct measurement of the embryo or fetus. Accuracy: about plus or minus 5 to 7 days, per ACOG, and considered the most accurate dating method overall. When it’s used: to confirm or, when needed, correct the LMP-based estimate.
  • Later ultrasounds — What it uses: measurements like head and abdomen size. Accuracy: lower than first-trimester dating, since growth varies more between babies later in pregnancy. When it’s used: mainly to track growth, not to reset the due date, once early dating is already established.

Why and When a Due Date Might Change

A due date isn’t fixed the moment it’s first mentioned. ACOG’s guidance lays out specific thresholds for when an ultrasound-based estimate should replace an LMP-based one, because how much the two can normally differ apart depends on how far along the pregnancy is when the ultrasound is done:

  • Up to 8 weeks 6 days — Change the due date if ultrasound and LMP dating differ by more than 5 days.
  • 9 weeks 0 days to 15 weeks 6 days — Change it if they differ by more than 7 days.
  • 16 weeks 0 days to 21 weeks 6 days — Change it if they differ by more than 10 days.
  • 22 weeks 0 days to 27 weeks 6 days — Change it if they differ by more than 14 days.
  • 28 weeks and beyond — Change it only if they differ by more than 21 days, since ultrasound is least precise for dating this late.

If the difference is smaller than the threshold for that stage, the original LMP-based due date usually stays. ACOG’s guidance also notes that a pregnancy without any ultrasound confirming or adjusting the due date before 22 weeks is considered suboptimally dated — one reason providers generally recommend getting an ultrasound earlier rather than later.

Extra-Caution Situations and Evidence Limits

Dating can be harder to pin down in certain situations, and it’s worth mentioning these to your provider directly:

  • Irregular, very long, or very short menstrual cycles, which make LMP-based dating less reliable on its own.
  • Uncertainty about the exact date your last period started.
  • Pregnancies conceived using fertility treatment, where conception timing may already be known more precisely than a typical LMP estimate.
  • A first ultrasound that doesn’t happen until the third trimester, where dating accuracy is lowest and any redating decision needs to weigh the full clinical picture.

This page covers general dating methods and does not calculate or interpret any individual due date. It also doesn’t replace the ultrasound report and clinical judgment your own provider uses, which take your full history into account.

A Quick Checklist for Your Dating Conversation

  • Write down the first day of your last menstrual period as precisely as you can, even if you’re not fully sure.
  • Mention whether your cycles are typically regular or irregular.
  • Ask when your first dating ultrasound will happen and why that timing was chosen.
  • Ask what method was used to calculate your due date and how confident that estimate is.
  • If your due date changes after an ultrasound, ask why, so you understand which estimate your care team is now using.

Frequently Asked Questions

Why did my due date change after my ultrasound?

Ultrasound measures the embryo or fetus directly, while LMP dating assumes an average cycle. When the two estimates differ by more than the amount expected for that stage of pregnancy, providers generally use the ultrasound-based date going forward, per ACOG guidance.

Is gestational age the same as how many weeks pregnant I “actually” am?

Gestational age is the standard clinical measurement, and it’s what’s used for due dates, prenatal visit schedules, and lab work. It typically runs about two weeks ahead of the time since conception, since it’s counted from your last period rather than from ovulation or fertilization.

What if I’m not sure when my last period started?

This is common and worth telling your provider directly rather than guessing silently. An early ultrasound can help establish or confirm gestational age even when the LMP date is uncertain.

Can I calculate my own accurate due date without an ultrasound?

You can estimate a due date from your LMP as a starting point, but ACOG’s guidance identifies first-trimester ultrasound as the most accurate way to confirm it. Only a clinician using your ultrasound and history can determine your actual estimated due date.

Educational Disclaimer

This article is for general educational purposes only and is not medical advice, diagnosis, or treatment. It does not calculate, confirm, or interpret any individual’s due date or gestational age. Reading it does not create a provider-patient relationship. Due date estimation and any changes to it should be discussed with a licensed healthcare provider who has your ultrasound results and full health history. If you are having a medical emergency, call 911 or go to the nearest emergency room. See our full Medical Disclaimer.