How each dating method is calculated
All five modes reduce to finding an "effective LMP" — the calendar date that a standard 280-day (40-week) pregnancy would have started from. Once that's known, GA on any date, EDD, and trimester all follow the same way.
GA by LMP
Naegele's Rule assumes a 28-day cycle with ovulation on day 14. A longer cycle means ovulation happens later relative to LMP, so the effective LMP — and everything calculated from it — shifts forward by the same number of days (and back for a shorter cycle). Leave cycle length at 28 for the classic, unadjusted rule.
GA by Scan
GA by IVF
GA on Date
Same mechanism as "GA by scan," generalised for any date where GA was already established — e.g. from an old report, a referral letter, or a prior visit — not necessarily an ultrasound.
EDD to LMP
Useful when only an EDD was given (e.g. by a previous provider) and you need the equivalent LMP or a current GA. The EDD you enter always stays the EDD reported back — cycle length here only back-calculates what raw LMP a patient with that cycle length would actually report, for charting purposes; it does not change the EDD or GA shown.
Trimester & term definitions
| Gestational age | Classification |
|---|---|
| 0 – 13w6d | 1st trimester |
| 14w0d – 27w6d | 2nd trimester |
| 28w0d – 40w+ | 3rd trimester |
| 37w0d – 38w6d | Early term |
| 39w0d – 40w6d | Full term |
| 41w0d – 41w6d | Late term |
| ≥ 42w0d | Post-term |
Term definitions per ACOG/SMFM (2013, reaffirmed) — replacing the older single "term" label of 37–42 weeks.
Week-by-week pregnancy milestones
General developmental milestones by gestational age (LMP-based convention). Individual pregnancies vary — this is an orientation reference, not a diagnostic timeline.
| Week | Milestone |
|---|---|
| 4 | Implantation complete; a gestational sac may be visible on transvaginal ultrasound by ~4.5–5 weeks. |
| 5 | Neural tube begins forming; primitive cardiac tube starts to develop. |
| 6 | Cardiac activity often detectable on transvaginal USG; embryo (CRL) roughly 4–6 mm. |
| 7 | Limb buds appear; brain vesicles forming. |
| 8 | Organogenesis well underway — the period of highest teratogenic sensitivity. |
| 9 | All major organ systems have begun to form; embryonic period drawing to a close. |
| 10 | Fetal period begins; fingers and toes separating. |
| 11 | Nasal bone ossification begins; external genitalia starting to form. |
| 12 | End of first trimester; nuchal translucency screening window (11–13w6d); CRL still used for dating. |
| 13 | Fetus can make sucking motions; vocal cords forming. |
| 14 | Second trimester begins; fetus can grimace; sex may be visible on ultrasound. |
| 15 | Skeleton beginning to ossify; limb movements increase. |
| 16 | Some multiparous women begin to feel movement ("quickening"). |
| 17 | Fat (adipose tissue) begins developing. |
| 18 | Anatomy/anomaly scan window opens (18–22 weeks); primigravidae often feel first movements. |
| 19 | Vernix caseosa begins coating the skin. |
| 20 | Halfway point; detailed anatomy scan typically performed; estimated weight ~300 g. |
| 21 | Fetus swallows amniotic fluid; eyebrows and eyelids formed. |
| 22 | Approaching the lower limit of viability; lanugo (fine hair) covers the body. |
| 23 | Pulmonary blood vessels developing; taste buds forming. |
| 24 | Viability threshold at most tertiary centres; surfactant production begins. |
| 25 | Nostrils begin to open; hands can form a fist. |
| 26 | Eyes begin to open; startle response to sound. |
| 27 | End of second trimester; rapid brain tissue growth. |
| 28 | Third trimester begins; eyes open and close; typical window for GDM screening (24–28 wks). |
| 29 | Muscles and lungs continue maturing; fetal head may begin engaging. |
| 30 | Fingernails present; amniotic fluid volume nearing its peak. |
| 31 | Rapid brain development; movement patterns more regular. |
| 32 | Toenails present; many (not all) fetuses now cephalic. |
| 33 | Bones hardening (except the skull, which stays pliable for delivery); immune system maturing. |
| 34 | Central nervous system maturing further; lungs approaching maturity. |
| 35 | Most physical development complete; rapid weight gain continues. |
| 36 | Baby often "drops" lower into the pelvis (lightening); most organs mature except lungs/brain. |
| 37 | Early term begins — generally considered ready for birth if labour starts. |
| 38 | Continued weight gain (~30 g/day); approaching full term. |
| 39 | Full term; lungs and brain reach final maturity. |
| 40 | Estimated due date. |
Fetal biometry by gestational week
Representative 50th-percentile (median) reference values — approximate, for quick orientation only. Use your ultrasound machine's validated growth chart (Hadlock or INTERGROWTH-21st) for actual percentile/centile interpretation and growth assessment.
Crown-Rump Length (CRL) — used for dating, 6–13w6d
| GA (weeks) | CRL (mm) |
|---|---|
| 6 | ~5 |
| 7 | ~10 |
| 8 | ~16 |
| 9 | ~23 |
| 10 | ~31 |
| 11 | ~41 |
| 12 | ~53 |
| 13 | ~66 |
BPD, HC, AC & FL — second/third trimester biometry, 14–40 weeks
| GA (wk) | BPD (mm) | HC (mm) | AC (mm) | FL (mm) |
|---|---|---|---|---|
| 14 | ~28 | ~105 | ~90 | ~13 |
| 16 | ~37 | ~133 | ~116 | ~22 |
| 18 | ~43 | ~154 | ~138 | ~28 |
| 20 | ~48 | ~175 | ~152 | ~32 |
| 22 | ~54 | ~195 | ~180 | ~37 |
| 24 | ~60 | ~221 | ~197 | ~44 |
| 26 | ~66 | ~240 | ~222 | ~49 |
| 28 | ~73 | ~255 | ~241 | ~53 |
| 30 | ~78 | ~275 | ~260 | ~58 |
| 32 | ~82 | ~289 | ~280 | ~62 |
| 34 | ~86 | ~305 | ~298 | ~66 |
| 36 | ~89 | ~317 | ~316 | ~69 |
| 38 | ~92 | ~330 | ~336 | ~73 |
| 40 | ~94 | ~342 | ~349 | ~76 |
Clinical use
- First-trimester dating is the most accurate — a CRL-based scan at 8–13w6d dates a pregnancy within about ±5–7 days. Accuracy worsens with advancing gestation (±2 weeks by the third trimester), because biological variation in fetal size grows over time.
- When LMP and scan disagree, ACOG/ISUOG guidance re-dates the pregnancy by ultrasound if the discrepancy exceeds the accepted threshold for that gestational age window (roughly: >5–7 days in the first trimester, >10 days in the early second trimester, >2 weeks in the third trimester) — otherwise LMP dating stands.
- IVF pregnancies should always be dated from the known transfer/retrieval date, not LMP — it's more precise than any ultrasound.
- Biometry beyond dating is used to assess fetal growth (e.g. detecting growth restriction or macrosomia) by comparing serial measurements against percentile charts — a different clinical purpose from establishing gestational age.
Frequently asked questions
What is an OB wheel / pregnancy wheel?
An OB wheel (obstetric or pregnancy wheel) is a dating tool — traditionally a physical cardboard disc, now usually digital — that converts a starting reference point (LMP, a scan date and GA, an IVF transfer date, or a known EDD) into gestational age, estimated due date, and trimester, without doing the day-counting by hand.
How accurate is an EDD calculated from LMP?
Naegele's Rule (LMP + 280 days) assumes ovulation on day 14 of a regular 28-day cycle. It's a reasonable population-level estimate but can be off by a week or more with irregular cycles, so it's superseded by first-trimester ultrasound dating whenever one is available.
What if my menstrual cycle isn't 28 days?
LMP-based dating becomes less reliable the further a cycle deviates from 28 days, because it assumes a fixed 14-day follicular phase. In these cases (and whenever LMP is uncertain), a first-trimester ultrasound is the preferred dating method.
What's the difference between GA by LMP and GA by scan?
GA by LMP counts forward from the reported last period. GA by scan back-calculates an "effective LMP" from a measured gestational age on ultrasound (e.g. via CRL). The two can disagree — ultrasound dating is generally trusted over LMP when the discrepancy is large enough, especially in the first trimester.
When is ultrasound dating most accurate?
Earlier is more accurate. A CRL-based first-trimester scan (8–13w6d) is the single most accurate dating method available, within roughly ±5–7 days. Second- and third-trimester biometry-based dating is progressively less precise.
How is GA calculated after IVF?
From the known embryo transfer date and embryo age at transfer (typically Day 3 or Day 5). Since egg retrieval corresponds to roughly LMP + 14 days in the obstetric dating convention, GA at transfer = embryo age + 14 days, and EDD = transfer date + 261 days (Day 5) or + 263 days (Day 3).
Can the EDD change during pregnancy?
The EDD is normally set once, at the most accurate dating opportunity available (ideally a first-trimester scan), and is not routinely moved later in pregnancy just because growth measurements differ — later-pregnancy biometry reflects fetal growth, not dating accuracy.
What's the difference between gestational age and fetal (embryonic) age?
Gestational age (the obstetric convention used throughout this tool) is counted from the LMP. Fetal or embryonic age is counted from conception/fertilization, which is about 2 weeks later — so fetal age is roughly GA minus 2 weeks.
How are trimesters and "term" defined?
Trimesters: 1st (0–13w6d), 2nd (14w0d–27w6d), 3rd (28w0d onward). "Term" is further split by ACOG/SMFM into early term (37w0d–38w6d), full term (39w0d–40w6d), late term (41w0d–41w6d), and post-term (≥42w0d).
Why do fetal biometry measurements (CRL, BPD, HC, AC, FL) matter?
Early on, CRL is the single most accurate measurement for dating a pregnancy. From the second trimester onward, BPD, HC, AC, and FL are combined to estimate fetal weight and, when tracked serially, to assess growth trend — flagging growth restriction or macrosomia — rather than to date the pregnancy.
References
- ACOG Committee Opinion No. 700: Methods for Estimating the Due Date. Obstet Gynecol. 2017;129:e150–e154.
- Committee on Practice Bulletins — Obstetrics & the Society for Maternal-Fetal Medicine. Definition of Term Pregnancy. Obstet Gynecol. 2013;122:1139–1140 (reaffirmed).
- Robinson HP, Fleming JE. A critical evaluation of sonar crown-rump length measurements. Br J Obstet Gynaecol. 1975;82:702–710.
- Hadlock FP, et al. Estimation of fetal weight with the use of head, body, and femur measurements. Am J Obstet Gynecol. 1985;151:333–337.
- Papageorghiou AT, et al. International standards for fetal growth based on serial ultrasound measurements: the INTERGROWTH-21st Project. Lancet. 2014;384:869–879.
- Practice Committee of the American Society for Reproductive Medicine (ASRM). Guidance on gestational age dating after IVF.
- ISUOG Practice Guidelines: performance of the routine mid-trimester fetal ultrasound scan. Ultrasound Obstet Gynecol. 2011;37:116–126.