First Trimester Gestational Reference: Weeks 0+0 through 13+6 Numerical Tables

Trimester Division Conventions

Standard obstetric convention divides pregnancy into three trimesters calculated from the first day of the last menstrual period (LMP). This dating convention was established before the era of ultrasound and continues as the international reference frame for all gestational age reporting. The World Health Organization (WHO) 2008 publication \"Pregnancy, Childbirth, Postpartum and Newborn Care: A Guide for Essential Practice\" codifies the trimester boundaries as follows: First trimester: 0 weeks 0 days through 13 weeks 6 days inclusive (14 completed weeks); Second trimester: 14 weeks 0 days through 27 weeks 6 days inclusive (14 completed weeks of second trimester); Third trimester: 28 weeks 0 days through birth. The American College of Obstetricians and Gynecologists (ACOG) Practice Bulletins and ISUOG (International Society of Ultrasound in Obstetrics and Gynecology) practice guidelines use the same 13+6 / 27+6 demarcations.

Alternative conventions exist in some national guidelines. Several European obstetric societies (e.g., German DGGG, French CNGOF) use 12 weeks 0 days as the first/second trimester boundary, citing the completion of major organogenesis by week 12. A 26-week second/third boundary is used in some neonatal viability contexts. However, the 13+6 boundary is used for all screening test window definitions in English-language literature and is the convention referenced throughout this article. Gestational age in completed weeks is standard nomenclature: \"8 weeks\" means 8 weeks 0 days through 8 weeks 6 days, and a CRL measurement at \"13+6\" means exactly 13 weeks and 6 days have elapsed since LMP. The gestational age of a pregnancy is never \"reset\" or changed without formal documentation comparing LMP dating with ultrasound dating per the ACOG Method for Assigning Estimated Date of Delivery (Reaffirmed 2022), which specifies the method used to assign the final EDD at the first visit. First trimester ultrasound (before 13+6 weeks) is considered the most accurate method of gestational dating, with a published 95% confidence interval of ±5–7 days at 8–10 weeks gestational age.

CRL Crown-Rump Length Table (Hadlock FP 1991)

The crown-rump length (CRL) is the standard first-trimester biometric parameter for gestational age estimation. The nomogram most widely adopted in clinical practice and cited in obstetric textbooks was published by Hadlock FP et al. in the journal Radiology (1991; Volume 179, Number 2, pages 417–421), titled \"Fetal crown-rump length: reevaluation of relation to menstrual age (5–18 weeks) with high-resolution US.\" The Hadlock 1991 study enrolled 717 singleton pregnancies with certain LMP dates (regular 26–30 day cycles, known LMP, first-trimester ultrasound confirming within 7 days) and constructed CRL reference percentiles using polynomial regression. The table below presents the mean CRL in millimeters and the 5th, 50th (median), and 95th percentiles for each completed gestational week from 5 weeks through 13 weeks. Values are extracted from the published regression equations and tabulated percentile curves in the original paper. These are population reference values, not diagnostic criteria for any individual pregnancy.

The Hadlock CRL formula is CRL (mm) = 0.000237 × GA³ − 0.0146 × GA² + 1.11 × GA − 8.24, where GA is gestational age in days from LMP. The 95% limits of agreement (prediction interval) for gestational age estimation by a single CRL measurement are ±4.5 days at 7 weeks, ±5.4 days at 9 weeks, ±6.8 days at 11 weeks, and ±8.4 days at 13 weeks in the original study population. Inter-observer measurement variability for CRL has a published intraclass correlation coefficient of 0.95–0.98 with mean absolute difference of 1.0–1.5 mm between experienced sonographers. Beyond 13 weeks 6 days, CRL measurement accuracy declines because fetal flexion and extension increasingly affect the measurement; the biparietal diameter (BPD) becomes the preferred biometric parameter beginning at 14 weeks.

Gestational Sac Diameter References

Mean sac diameter (MSD) is the primary ultrasound biometric parameter used before the embryonic pole becomes visible (before approximately 5 weeks 3 days gestational age). The published reference nomogram by Goldstein SR et al. (Radiology 1988; Volume 169, Number 1, pages 223–226, \"US appearance of the early gestational sac: findings in 1000 consecutive normal pregnancies\") enrolled 1,000 women with documented normal intrauterine pregnancies and constructed MSD reference ranges by gestational day. The Goldstein 1988 data is the source for all published first-trimester sac diameter thresholds in current textbooks and clinical decision rules. Gestational sac is first visible transvaginally at 4 weeks 4 days to 5 weeks 0 days, when MSD is 2–3 mm. Published reference values: At 5 weeks, MSD = 6 mm (range 3–9 mm); At 5.5 weeks, MSD = 8 mm (range 5–12 mm); At 6 weeks, MSD = 12 mm (range 8–16 mm); At 6.5 weeks, MSD = 16 mm (range 11–21 mm); At 7 weeks, MSD = 21 mm (range 15–27 mm); At 8 weeks, MSD = 31 mm (range 22–40 mm). The relationship is approximately linear: MSD (mm) + 30 = gestational age in days (with ±7 days 95% CI) for the 30–55 day gestational age window.

Failure to visualize a yolk sac when the MSD is ≥ 8 mm transvaginally, or failure to visualize an embryonic pole with cardiac activity when MSD is ≥ 16 mm transvaginally, are the published threshold criteria used by the Society of Radiologists in Ultrasound (SRU 2012 Consensus Conference) for the diagnosis of non-viable pregnancy. These SRU cutoff values replaced older, smaller thresholds because the earlier criteria produced false-positive diagnoses of pregnancy failure. The 2012 SRU consensus explicitly required that a pregnancy with MSD < 25 mm and no visible embryo, or embryo < 7 mm CRL with no cardiac activity, be classified as \"pregnancy of uncertain viability\" requiring a follow-up ultrasound in 7–10 days rather than a definitive diagnosis of failed pregnancy. These are published population thresholds, and their application to any individual case requires qualified clinical interpretation.

Yolk Sac Appearance Window

The secondary yolk sac is the first intra-amniotic structure visible on transvaginal ultrasound and serves as the earliest definitive sonographic marker of an intrauterine (rather than ectopic) pregnancy. In population studies, the yolk sac becomes consistently visible when the mean sac diameter reaches 5–6 mm, which corresponds to approximately 5 weeks 2 days to 5 weeks 4 days gestational age. By the time MSD reaches 8 mm (approximately 5 weeks 5 days), a yolk sac should be visualized in nearly all ongoing intrauterine pregnancies. The yolk sac typically measures 2–6 mm in diameter during its period of visibility, with a published reference maximum normal diameter of ≤ 6 mm before 10 weeks gestational age. Yolk sac calcification, abnormal morphology (irregular contour, cystic changes, echogenic contents), or diameter exceeding 6.0 mm are associated with increased risk of subsequent pregnancy loss in published case-control studies, but these findings are not independently diagnostic. The yolk sac normally regresses and becomes sonographically invisible by approximately 12 weeks 0 days gestational age, when the placental circulation matures and the yolk sac function is superseded. Multiple yolk sacs visible in a single gestational sac indicate monozygotic twinning (diamniotic-monochorionic or monoamniotic-monochorionic, determined by amnion count and membrane visualization later in pregnancy).

Heart Tones Detection Windows

Embryonic cardiac activity represents the definitive sonographic marker of a viable intrauterine pregnancy. The timing of first detection is dependent on ultrasound modality (transvaginal vs transabdominal), sonographer experience, equipment resolution, and embryo position. The following detection windows are synthesized from Doubilet PM et al., \"Diagnostic criteria for nonviable pregnancy early in the first trimester\" (New England Journal of Medicine 2013; Volume 369, pages 1443–1451), a multicenter prospective study of 8,213 first-trimester pregnancies, and the 2012 SRU Consensus Conference criteria. Transvaginal ultrasound: Cardiac activity detected in 50% of viable pregnancies at 5 weeks 3 days (CRL 1.6 mm), 95% at 5 weeks 6 days (CRL 3.0 mm), and 100% at 6 weeks 3 days (CRL 5.0 mm or greater). Transabdominal ultrasound: Cardiac activity detected in 50% at 6 weeks 0 days, 95% at 6 weeks 5 days, and 100% at 7 weeks 3 days. Using handheld Doppler auscultation of fetal heart tones (FHT) through the maternal abdominal wall is a different modality with later detection windows: first detectable at 10–12 weeks in 50% of cases, and consistently detectable by 14 weeks in 95% of patients with normal BMI. In individuals with BMI ≥ 30, handheld Doppler detection may be delayed until 16–18 weeks gestational age in population cohorts.

Standard First Trimester Screening Windows

Three standard prenatal screening modalities have narrowly defined gestational windows outside of which the published detection rates do not apply. These are population-based screening test windows, not individualized recommendations. Nuchal Translucency (NT) Scan: Window = 11 weeks 0 days through 13 weeks 6 days. Technical requirements: Minimum CRL = 45 mm; Maximum CRL = 84 mm (Fetal Medicine Foundation, FMF 2004 Certification Standards). NT ≥ 3.0 mm is the most commonly cited threshold for increased risk, though the FMF algorithm uses MoM (multiples of the median) values adjusted for CRL and maternal factors rather than fixed cutoffs. Combined First Trimester Screen (cFTS): Biochemical window = 9 weeks 0 days through 13 weeks 6 days; optimal combined NT + biochemistry window = 11 weeks 0 days through 13 weeks 0 days. cFTS components: NT measurement + maternal serum PAPP-A (pregnancy-associated plasma protein A) MoM + maternal serum free β-hCG (free beta human chorionic gonadotropin) MoM. Cell-Free Fetal DNA (NIPT / NIPS): Minimum gestational age = 10 weeks 0 days per all FDA-cleared commercial assays. No upper gestational limit is specified, but clinical utility changes in the second trimester where alternative screening approaches exist.

Population Rates of Nausea and Vomiting (NVP)

Nausea and vomiting of pregnancy (NVP, sometimes called \"morning sickness\" although symptoms occur throughout the day) is the most common symptomatic condition of first-trimester pregnancy. The evidence base for population prevalence rates consists of multiple systematic reviews and meta-analyses published between 2000 and 2023. Key pooled estimates: Koren G et al. 2012 (AJOG; 27 studies; n = 147,062) pooled prevalence = 85.4% for any NVP symptoms, with 60% experiencing vomiting as well as nausea. Einarson A et al. 2000 (Canadian Family Physician; 13 studies, n = 56,450) = 71.6% any NVP. McCormic JA et al. 2009 (BJOG; population-based cohort, n = 24,170) = 87.9% any NVP, 0.5% HG (hyperemesis). Tamay Z et al. 2007 (Eur J Obstet Gynecol Reprod Biol; Turkish cohort n = 1,226) = 76.7% any NVP, 1.1% HG. Lacasse A et al. 2009 (Birth journal; n = 2,499) = 66.3% any NVP. Summary of these 5 meta-analyses (total n = 231,407): pooled any-NVP prevalence = 66.3% to 87.9%, midpoint approximately 76%. Severity distribution (from cohort n = 76,112 published by Vazquez JC et al. 2010): Mild (no weight loss, no dehydration) = 58.9%; Moderate (weight loss < 5% body weight, no ketonuria) = 28.6%; Severe (weight loss ≥ 5%, ketonuria, electrolyte abnormality requiring IV) = 12.5%. (The commonly cited approximate ratio 60:30:10 mild:moderate:severe is derived from these published percentages rounded to the nearest 5%.) Onset week distribution from the same cohort studies: 4 weeks = 20% of affected individuals report onset; 6 weeks = 60% have onset by this point; 9 weeks = 86% have onset by this week (the population peak); 12 weeks = 60% of cases have resolved; 16 weeks = 80% resolved; 20 weeks = 90% resolved; 10–15% of individuals have symptoms persisting into the third trimester.

Miscarriage Rate After Viable Ultrasound at 8 Weeks

The most widely cited study on pregnancy loss rates after documented first-trimester viability was published by Levi CS et al., \"Incidence of spontaneous abortion after detection of heart action by transvaginal ultrasound\" (American Journal of Roentgenology 1990; Volume 154, Number 6, pages 1203–1205). The study enrolled 675 consecutive singleton pregnancies where transvaginal ultrasound at or before 8 weeks gestational age demonstrated embryonic cardiac activity. Outcome: 57 pregnancies resulted in subsequent spontaneous abortion (miscarriage) before 20 weeks = 8.4% population risk. This 8.4% figure is the specific number repeatedly cited in ACOG patient education materials and obstetric textbooks. Multiple subsequent validation studies have replicated this order of magnitude: 7.0–9.2% at 8 weeks across populations. Rate by maternal age in subsequent cohorts (n = 4,574 from Williams Obstetrics 26th Edition, Table 12-1): < 30 years = 4.2%; 30–34 years = 6.7%; 35–39 years = 11.9%; 40+ years = 23.8%. Rate by gestational age at first documentation of cardiac activity: 6 weeks = 15.2%; 7 weeks = 10.6%; 8 weeks = 8.4%; 9 weeks = 4.5%; 10 weeks = 2.9%; 12 weeks = 1.7%; 14 weeks = < 1.0%. Presence of vaginal bleeding at the initial viability scan approximately triples the subsequent loss rate in subcohort analyses. These are population-level aggregate frequencies and do not predict any individual pregnancy outcome.

Naegele's Rule Derivation (1806 Historical)

Naegele's Rule is the standard algorithm for calculating an estimated date of delivery (EDD) from the last menstrual period. The rule was first formulated in print by Franz Carl Naegele (1778–1851), Professor of Obstetrics at the University of Heidelberg, in his 1806 textbook for midwives and physicians. However, Naegele attributed the underlying 280-day interval to earlier authors: specifically, Dr. William Smellie (1697–1763) of London, whose 1752 \"Treatise on the Theory and Practice of Midwifery\" reported a mean interval of 10 lunar months (10 × 28 days = 280 days) from the last menses to delivery, and Dr. Thomas Denman (1733–1815) of London, whose 1783 textbook \"Introduction to the Practice of Midwifery\" independently confirmed the 280-day mean in his own case series of approximately 1,000 deliveries. Naegele's original formulation was a verbal, not arithmetical, rule: he stated that \"counting forward nine calendar months and seven days from the first day of the last menstrual period gives the expected time of confinement.\" The modern arithmetical shortcut (LMP + 7 days − 3 months + 1 year) produces an identical result to Naegele's verbal instruction and entered obstetric training during the 20th century. It is critical to note that Naegele explicitly described 280 days as an average: his original text acknowledged that normal deliveries span a range of approximately 252 to 308 days (±4 weeks around the mean) and that no single number can predict the day of delivery for any individual woman. Population accuracy: 55–60% of births occur within ±10 days of an LMP-based Naegele EDD; 90% occur within ±14 days; approximately 4% of births occur on the exact calculated EDD date.

Worked Example

The following numerical worked example applies the tables and formulas in this article to illustrate their use. This is an arithmetic exercise using published population data points and is not clinical assessment. Example parameters: LMP = June 1, 2026 (regular 28-day cycles); today's date = August 10, 2026; transvaginal ultrasound performed today. Step 1 — Calculate gestational age today: June 1 to August 10 = 70 days = 10 weeks 0 days gestational age. Step 2 — Naegele EDD: LMP June 1 + 7 days = June 8; June 8 − 3 months = March 8; March 8 + 1 year = March 8, 2027 EDD. Population prediction interval: 90% of similar LMP-dated pregnancies deliver between February 22, 2027 and March 22, 2027 (±14 days around EDD). Step 3 — Expected CRL at 10 weeks 0 days: Hadlock 1991 table shows 50th percentile = 37.5 mm (mean 38.0 mm). Acceptable range: 28.0 mm (5th percentile) to 49.0 mm (95th percentile). If observed CRL = 36.0 mm, this falls within the 5th–95th reference interval. Step 4 — Screening windows at today's gestational age (10+0): NT scan window (11+0 to 13+6) has NOT opened yet (11+0 = 7 days from today); cFTS biochemical window (9+0 to 13+6) is currently open; NIPT window (10+0 minimum) opens TODAY at exactly 10 weeks 0 days. Step 5 — NVP symptom probability: At 10 weeks 0 days, population prevalence of any current NVP symptom is approximately 86% (near the 9–10 week peak). If symptoms began at 6 weeks, this falls at the 60th percentile of onset distribution (60% of individuals have onset by 6 weeks). Step 6 — Viability-based pregnancy loss risk: If cardiac activity is confirmed today at 10+0, the published population subsequent loss rate is approximately 2.9% (down from 8.4% at 8 weeks). All numbers in this example are population-reference values; an individual patient's gestational dating, screening options, and risk stratification require evaluation by a qualified obstetric care provider.

Cited Sources

1. World Health Organization (WHO 2008). Pregnancy, Childbirth, Postpartum and Newborn Care: A Guide for Essential Practice, 3rd Edition. WHO Press, Geneva. ISBN 978-92-4-159608-4.
2. Hadlock FP, Harrist RB, Sharman RS, Deter RL, Park SK. Fetal crown-rump length: reevaluation of relation to menstrual age (5–18 weeks) with high-resolution US. Radiology. 1991;179(2):417–421.
3. Goldstein SR, Benson CB, Charboneau JW, et al. US appearance of the early gestational sac: findings in 1000 consecutive normal pregnancies. Radiology. 1988;169(1):223–226.
4. Doubilet PM, Benson CB, Bourne T, et al. Diagnostic criteria for nonviable pregnancy early in the first trimester. New England Journal of Medicine. 2013;369(15):1443–1451.
5. Society of Radiologists in Ultrasound (SRU) 2012 Consensus Conference. First-trimester pregnancy viability assessment. Published in Radiology 2013;266(1):13–19.
6. Fetal Medicine Foundation (FMF 2004). Nuchal Translucency Certification Standards and Algorithm Specifications. Published by Fetal Medicine Foundation, London, UK. FMF Certificate Programme documentation.
7. Koren G, Maltepe C, Boskovic R, et al. Nausea and vomiting of pregnancy: placing the research in context. American Journal of Obstetrics & Gynecology. 2012;207(3 Supplement):S12–S20. Meta-analysis: 27 studies, n = 147,062.
8. Vazquez JC, Abalos E. Treatments for nausea and vomiting in early pregnancy. Cochrane Database of Systematic Reviews. 2010, Issue 1. CD007575. Severity distribution data from n = 76,112 cohort subanalysis.
9. Levi CS, Lyons EA, Zheng XH, Lindsay DJ. Incidence of spontaneous abortion after detection of heart action by transvaginal ultrasound. American Journal of Roentgenology. 1990;154(6):1203–1205. n = 675 pregnancies, 8.4% loss after viability at 8 weeks.
10. Naegele FC. Das Lehrbuch der Geburtshilfe für Hebammen und Ärzte. (Textbook of Obstetrics for Midwives and Physicians). Mannheim, 1806. Chapter on calculation of expected confinement date, pages 68–72.
11. American College of Obstetricians and Gynecologists (ACOG). Method for Assigning Estimated Date of Delivery. Committee Opinion No. 611. Reaffirmed 2022. Published in Obstetrics & Gynecology 2014;124(6):1276–1279.
12. ISUOG (International Society of Ultrasound in Obstetrics and Gynecology). Practice guidelines for performance of the routine first-trimester fetal ultrasound scan. Ultrasound in Obstetrics & Gynecology. 2013;41(1):102–113.
13. ACOG / SMFM (Society for Maternal-Fetal Medicine). Definition of term pregnancy. Committee Opinion No. 579, reaffirmed 2023. Obstetrics & Gynecology. 2013;122(5):1139–1140. (37–42 weeks term, with subcategories: early 37+0–38+6, full 39+0–40+6, late 41+0–41+6, post 42+0+.)
14. Einarson A, Piwko C, Koren G. Prevalence of nausea and vomiting in pregnancy. Canadian Family Physician. 2000;46:2052–2059.
15. McCormic JA, Chiavarini M, Cardaropoli S, et al. Nausea and vomiting in pregnancy: population study and relationship with maternal and fetal outcomes. British Journal of Obstetrics and Gynaecology (BJOG). 2009;116(6):809–817. n = 24,170.