If you have had your first prenatal ultrasound, your sonographer probably handed you a printout with a “gestational age” number on it. Maybe it matched what you expected from your last menstrual period; maybe it differed by a few days, or even a week. This guide walks through how that number is calculated, why it usually wins over LMP-based dating, and when to use it instead.
The basics: why we date pregnancy at all
Pregnancy dating drives most of the timeline of prenatal care. The viability scan happens around week 6–8. Genetic screening (NIPT) opens at week 10. The nuchal translucency scan is week 11–14. The anatomy scan is week 18–22. Glucose screening is around week 24–28. GBS testing is around week 35–37. Induction or scheduled C-section, if needed, is timed against the due date. The whole calendar of prenatal care depends on knowing how pregnant you are, accurately.
Two ways to date a pregnancy
There are two methods clinicians use to assign a due date:
- LMP-based dating. Add 280 days (40 weeks) to the first day of your last menstrual period. This is Naegele’s rule, dating from 1812 and remarkably durable.
- Ultrasound dating. Measure something on the embryo or fetus that grows at a predictable rate. Most often, this is the crown-rump length (CRL) — the distance from the top of the embryo’s head to the bottom of its rump.
These two methods often give different answers. The question is: which one wins?
LMP dating, and where it fails
LMP dating assumes a 28-day cycle with ovulation on day 14. If your cycle is 28 days, you ovulated on day 14, you conceived that day, and 266 days later (38 weeks from conception, 40 weeks from LMP) you give birth — the math is internally consistent.
The problem is that real cycles vary. If your cycle is 35 days, you probably ovulated around day 21, which means LMP-based dating overestimates your gestational age by about a week. If your cycle is 24 days, you may have ovulated on day 10, and LMP underestimates by several days. PCOS users, post-pill users, and anyone with irregular cycles may have LMP-based dates that are off by ten days or more. And if you simply do not remember the exact date your last period started, the whole calculation is shaky from the beginning.
CRL dating: how it works
In the first trimester, embryos and early fetuses grow at a remarkably consistent rate that does not depend much on individual factors. Measuring the CRL on ultrasound gives a number you can plug into a formula:
GA (in days) = 8.052 × √CRL(mm) + 23.73
This is the original Robinson-Fleming formula (BJOG 1975), still the foundation of first-trimester dating. Modern formulae from Hadlock, Verburg, and others refine the calculation slightly but produce numbers within a day or two of Robinson-Fleming for the same CRL.
A few examples:
- CRL 10 mm → GA ≈ 49 days = 7 weeks 0 days
- CRL 30 mm → GA ≈ 68 days = 9 weeks 5 days
- CRL 50 mm → GA ≈ 81 days = 11 weeks 4 days
- CRL 65 mm → GA ≈ 89 days = 12 weeks 5 days
The sonographer measures the CRL on the screen, the ultrasound machine looks up the corresponding gestational age from a built-in table, and prints the result.
The 7-day rule: when ultrasound wins
ACOG Committee Opinion 700 (2017) gives the canonical rule for resolving discrepancies between LMP-based and ultrasound-based dating:
| Gestational age at first ultrasound | Threshold for ultrasound to win |
|---|---|
| Up to 8 weeks 6 days | More than 5 days difference |
| 9 weeks 0 days to 13 weeks 6 days | More than 7 days difference |
| 14 weeks 0 days to 15 weeks 6 days | More than 7 days difference |
| 16 weeks 0 days to 21 weeks 6 days | More than 10 days difference |
| 22 weeks 0 days to 27 weeks 6 days | More than 14 days difference |
| 28 weeks 0 days and beyond | More than 21 days difference |
The pattern: the earlier the ultrasound, the tighter the threshold for it to override LMP. By the third trimester, fetal growth is variable enough that ultrasound-based dating is no longer reliable, and earlier dating estimates take precedence.
In practice, your clinician is the one applying this rule. They look at your LMP, your first ultrasound, the GA gap, and the rule above, and assign an “estimated due date” (EDD) that becomes your official date for the rest of the pregnancy.
What if my LMP and ultrasound match?
Good — that is the expected outcome for most users. A 1–3 day difference in the first trimester is normal and within the precision of both methods. The ultrasound effectively confirms the LMP date.
What if I do not know my LMP?
This is more common than you might think. Maybe you were on hormonal contraception that suppressed your normal cycle. Maybe you were post-partum and not yet menstruating regularly. Maybe you conceived during your first cycle off birth control without tracking. Maybe you simply cannot remember.
In all these cases, ultrasound dating is the only reliable method. The CRL gives a precise gestational age regardless of whether you can date your last period. This is one of the strongest arguments for getting a first-trimester scan early — even if cost or insurance coverage means you have to skip later optional scans.
Special case: IVF pregnancies
If you conceived through assisted reproduction, the conception date is known exactly and dating is the most precise of all. Day 5 transfer adds 261 days to find the due date; Day 3 adds 263; Day 6 adds 260. This is the gold standard — even ultrasound CRL dating is checked against the IVF date and rarely overrides it. Use IVF mode in our Due Date Calculator or Pregnancy Week Calculator to compute the right number.
What you can do with this
A few practical takeaways:
- Bring your LMP and ultrasound notes to every prenatal appointment. If your clinician switches your dating, ask why and what your new due date is. A 5-day shift changes some scheduling but nothing fundamental about the pregnancy.
- Note the GA on your printout, not just the date. “9 weeks 4 days” is more useful than “due date of December 12” because it lets you cross-reference any future scan or appointment.
- If you have irregular cycles or PCOS, push for an early scan. First-trimester ultrasound before 14 weeks is the most reliable dating method available, and ACOG recommends it routinely in any case where LMP is uncertain.
The bottom line
Modern obstetric care has moved decisively toward ultrasound dating because the math is more precise than memory and Naegele’s 1812 rule. But the LMP still matters — it is the anchor for every other prediction, and a wildly inconsistent LMP is itself useful clinical information. Use both. Plug your scan results into our Pregnancy Week Calculator or Due Date Calculator — both support ultrasound-based and IVF-based dating modes, not just LMP — to keep your numbers aligned with what your clinician is using.