What is gestational age and how is it calculated?

Gestational age (GA) is the time elapsed since the first day of the Last Menstrual Period (LMP). It is calculated as: GA = (today − LMP) ÷ 7 days. A 40-week pregnancy has 280 days from LMP to estimated due date (EDD). Gestational age is reported as "weeks + days" (e.g., 32 4/7 weeks = 32 weeks 4 days). The LMP convention is the standard in obstetrics, compared to embryology where post-conception age = GA − 14 days.

What are the trimesters and when does each occur?

The three trimesters are: First Trimester (1–13 wks gestation, weeks 1–13 post-LMP), Second Trimester (14–27 wks), Third Trimester (28–40+ wks). TOD the boundaries are clinical conventions; the actual physiological transitions (e.g., miscarriage risk drop, viability threshold) occur continuously. The first trimester is associated with highest miscarriage risk; the second trimester has lowest pregnancy-loss risk and is when most fetal anatomy develops; the third trimester is the final growth and maturation phase.

What is the first trimester combined screening test?

The first-trimester combined screening test, performed at 11–13 weeks 6 days gestation, combines: nuchal translucency ultrasound (NT > 3.0 mm is elevated), serum PAPP-A (low PAPP-A associated with trisomy 21), serum free β-hCG (high β-hCG associated with trisomy 21), and maternal age. Detection rate for trisomy 21 is 85–90% at a 5% false-positive rate. Combined with second-trimester quad test, detection rate rises to 95%. NIPT (cell-free fetal DNA) can detect trisomy 21 with >99% sensitivity from week 10 onward.

What is viability and when does it occur?

Fetal viability refers to the gestational age at which the fetus has reasonable probability of survival outside the womb with medical support. The viability threshold is generally ~24 weeks in industrialised countries with full NICU support, though some reports document survival at 22–23 weeks in expert centres. ACOG does not mandate a specific cutoff but considers 24 weeks the practical threshold for aggressive intervention. Below 22 weeks, survival without major morbidity is <5%. Between 22–24 weeks, survival ranges 5–40% with high morbidity. Above 24 weeks, survival rates rise rapidly.

What is the anatomy ultrasound and what does it check?

The anatomy ultrasound (anomaly scan) is performed at 18–22 weeks and evaluates fetal anatomy: brain (including ventricles for hydrocephalus), face (cleft lip/palate), spine (spina bifida), heart (4-chamber view, outflow tracts), kidneys (hydronephrosis), abdomen (omphalocele, gastroschisis), limbs (skeletal dysplasias), and placental location. Major structural anomalies are detected in ~3% of pregnancies. Experienced sonographers identify ~95% of major anomalies in low-risk pregnancies when performed with modern high-resolution equipment.

What is the preterm/late-term classification?

ACOG 2013 redefined term pregnancy. Preterm: <37 weeks (a 30–35% rate of NICU admission). Early Term: 37–38 6/7 wks (slightly higher morbidity than 39–40). Full Term: 39 0/7–40 6/7 wks (optimal delivery window, lowest morbidity). Late Term: 41 0/7–41 6/7 wks (when induction often offered). Post-Term: ≥42 weeks (mandatory induction/c-section; rising mortality without intervention). Modern practice aims to delay elective inductions to 39+0 wks.

What is the most accurate pregnancy dating method?

The first-trimester ultrasound crown-rump length (CRL) measurement at 8–13 weeks is the most accurate method: ±5 days at <9 wks, ±7 days at 9–14 wks. ACOG 2013 recommends ultrasound-dated EDD if CRL-based EDD differs from LMP-based EDD by more than the threshold (5 days for CRL <9 wks). Second-trimester biparietal diameter (BPD) ±10 days; later biometrics are progressively less accurate. LMP is reliable only when the menstrual cycle was regular, the date was accurately known, and there was no hormonal contraception in the cycle before pregnancy.

When does fetal movement (quickening) start?

First fetal movement (quickening) typically begins around 16–18 weeks for experienced mothers (second pregnancy or later) and 18–22 weeks for first-time mothers. The first sensations are often described as "flutters" or "butterflies," later developing into distinguishable kicks and rolls. By 24 weeks, fetal movement should be established. By 28 weeks, daily fetal movement counts are typically recommended (10+ movements in 2 hours is the standard threshold). Reduced fetal movement is often the first indication of fetal distress and warrants prompt evaluation.

How often do I see the doctor during pregnancy?

Standard prenatal visit schedule: weeks 4–28 monthly; weeks 28–36 bi-weekly; weeks 36–40+ weekly; post-40 weeks 2x weekly. High-risk pregnancies (multiple gestation, gestational diabetes, hypertension) typically have more frequent visits. Ultrasound visits are scheduled at 8–13 wks (dating), 18–22 wks (anatomy), 28–32 wks (growth), and additional scans as needed for indications.

What is the difference between gestational age and fetal age?

In clinical obstetrics, gestational age (GA) tracks from LMP (e.g., 38 weeks). In embryology and fetal development research, post-conception age (or fetal age, embryonic age) tracks from conception (e.g., 38 weeks GA = 36 weeks post-conception = 36 fetal weeks). All clinical gestational milestones use GA. To convert: fetal age = GA − 14 days.

Key Takeaways

→Gestational age is calculated from the first day of the Last Menstrual Period (LMP) and progresses as weeks + days. A 40-week pregnancy has 280 days from LMP to estimated due date (EDD). Pregnancy is conventionally divided into 3 trimesters: First Trimester (1–13 wks), Second Trimester (14–27 wks), Third Trimester (28–40+ wks). The ACOG 2013 term pregnancy classification further subdivides the third trimester: Early Term (37 0/7 to 38 6/7), Full Term (39 0/7 to 40 6/7), Late Term (41 0/7 to 41 6/7), Post-Term (≥42 0/7).
→First-trimester fetal development is rapid and critical: Week 4 (positive pregnancy test, hCG detectable), Week 5 (yolk sac visible on ultrasound), Week 6 (heartbeat begins at 110–170 bpm, embryo ~6 mm), Week 8 (major organ systems formed, transition from embryo to fetus), Week 10 (NIPT possible from maternal blood), Week 12 (end of first trimester; miscarriage risk drops below 1%). The first-trimester screening (nuchal translucency + serum markers PAPP-A, free beta-hCG) is typically done at 11–13 weeks.
→Second-trimester milestones include the anatomy ultrasound at 18–22 weeks (US standard is 20 weeks), which evaluates all major structural anatomy and often reveals fetal sex. Movement begins ~16 weeks for experienced mothers, ~20 weeks for first-time mothers. Week 24 is the typical viability threshold with NICU support. Miscarriage risk falls below 0.5% from week 12 onward. Preterm delivery risk begins to emerge in late second trimester, particularly for high-risk pregnancies.
→Third-trimester care intensifies. At 28 weeks, prenatal visits become bi-weekly if high-risk or weekly from 36 weeks onward. Week 32 marks the common preterm window with fetal movement counts becoming standard. Week 34 often sees lung maturity completion, with corticosteroid series beneficial if preterm delivery seems imminent by 7 days. Week 36 begins weekly visits and cervical checks are typical. Week 37 is "early term"; week 39 opens the optimal delivery window ("full term" per ACOG 2013).
→The first-trimester ultrasound is the most accurate dating method: crown-rump length (CRL) at 7–12 weeks has ±5 days accuracy; CRL at 9–14 weeks has ±7 days; biparietal diameter (BPD) at 14–22 weeks has ±10 days. ACOG Committee Opinion 579 (2013, reaffirmed 2020) recommends ultrasound-dated EDD if it differs from LMP-dated EDD by more than 5 days (CRL <9 wks), 7 days (CRL 9–14 wks), or 10 days (BPD 14–22 wks). Modern OB-GYN practice uses ultrasound-dated EDD as the primary reference in most pregnancies.

Pregnancy Week: The Clinical Timeline of Gestational Age

The standard pregnancy timeline used by every OB-GYN worldwide traces back to Franz Karl Naegele's 1812 Heidelberg publication "Die Lehre von der Schwangerschaft" (The Doctrine of Pregnancy). Naegele's Rule — Expected Date of Delivery (EDD) = LMP + 7 days + 9 months = LMP + 280 days — became the foundation of pregnancy dating. ACOG (American College of Obstetricians and Gynecologists) refined the timeline in 2013 with Committee Opinion 579, defining subcategories of "term pregnancy" that distinguish the optimal delivery window (39 0/7 to 40 6/7 weeks) from earlier ("early term" 37 0/7 to 38 6/7) and later ("late term" 41 0/7 to 41 6/7) terms. This 2013 update reshaped obstetric practice to deliberately delay scheduled deliveries until 39 weeks when medically possible, reducing neonatal morbidity by approximately 20% in the United States.

Gestational age — the LMP-based timeline
First trimester (1–13 weeks) — embryonic & early fetal period
Second trimester (14–27 weeks) — anatomy + movement
Third trimester (28–40+ weeks) — final maturation
ACOG 2013 term pregnancy subcategories
Prenatal care schedule (CDC + ACOG recommendations)
Ultrasound dating accuracy by gestational week
Frequently Asked Questions

Gestational age — the LMP-based timeline

Gestational age (GA) is calculated as:

GA = (today − LMP first day) / 7 days

For example, a woman whose LMP was January 1 and delivers in October 8 (280 days later) has a 40-week pregnancy. Modern gestational ages are reported as "weeks + days" — e.g., 32 4/7 weeks means 32 weeks and 4 days. The fraction notation reflects the day-precision minus the day-multiple of 7 (4 days = 4/7 weeks).

Why LMP convention? Because (1) it's the only universally recorded date, (2) histological gestational age (estimated from fetal measurements in early ultrasound) correlates well with LMP-based age when LMP is well-remembered, and (3) before ultrasound dating (which became routine in the 1970s–1980s), LMP was the only reference. The "LMP convention" is obstetric shorthand; embryologists use "post-conception age" or "embryonic age," which is LMP-based gestational age minus 14 days.

First trimester (1–13 weeks) — embryonic & early fetal period

The first 13 weeks see the most rapid developmental changes. Below are the standard clinical milestones:

First-Trimester Developmental Milestones

Week	Milestone	Clinical event
1	LMP / pre-implantation	Period begins, ovulation at end of week 2 typically
2	Ovulation + conception	Fertilisation in fallopian tube
3	Implantation (6–12 days post-fertilisation)	Embryo begins to secrete hCG
4	Positive pregnancy test	hCG detectable in urine (~25 mIU/mL)
5	First ultrasound	Yolk sac visible, intrauterine pregnancy confirmed
6	Heartbeat begins	Fetal cardiac activity 110–170 bpm, embryo ~6 mm
7	Craniofacial features visible	Nasal pits, eye lenses, brain hemispheres forming
8	Embryonic to fetal transition	Major organ systems formed, longer-limbs, fingers, toes
9	First-trimester hormones peak	hCG peak (~100,000 mIU/mL); morning sickness max
10	NIPT possible	Non-invasive prenatal testing via cell-free fetal DNA
11	First-trimester screening window	Nuchal translucency ultrasound + serum PAPP-A + free β-hCG
12	End of first trimester	Miscarriage risk <1% (down from ~15% at week 4)
13	NT scan optimal	NT scan completion window closes end of week 13+6 days

The first-trimester screening (sometimes called "combined test") at 11–13+6 weeks has a 85–90% detection rate for trisomy 21 (Down syndrome) when combining NT measurement with serum markers and maternal age. Non-invasive prenatal testing (NIPT) via cell-free fetal DNA can detect trisomy 21 with 99% sensitivity from week 10 onward, but is typically offered after the first-trimester combined screen returns elevated risk.

Second trimester (14–27 weeks) — anatomy + movement

The second trimester is often called the "golden period" because morning sickness subsides, energy returns, and major anatomical development is complete by week 20. Key milestones:

Second-Trimester Developmental Milestones

Week	Milestone	Clinical event
14	Second trimester begins	Risk of miscarriage drops below 0.5%
16	Quickening begins	First fetal movement (may feel like butterflies)
17–19	Fetal growth acceleration	Fundal height starts becoming measurable
18–22	Anatomy ultrasound window	Major structural review (anatomy screen); can determine sex
20	Anatomy scan optimal	Spanish/ACOG recommends 18–22 weeks; most done at 20
22	Viability cusp (with NICU support)	~22% survival with full intervention
23	Viability threshold high-income countries	35–55% survival with NICU
24	Standard viability threshold	50–70% survival with NICU support
26	Surfactant begins	Fetal lung surfactant production begins
27	End of second trimester	Weighs ~1.0 kg; viable with NICU

Second-trimester screening options include the quadruple test (AFP, hCG, estriol, inhibin-A) at 15–22 weeks, which detects ~80% of neural tube defects and ~75% of trisomies when combined with first-trimester results.

Third trimester (28–40+ weeks) — final maturation

The third trimester sees accelerated fetal growth (200g to 300g gained per week from 28 to 38 weeks), lung maturation, and brain development. Standard milestones:

Third-Trimester Developmental Milestones

Week	Milestone	Clinical event
28	Third trimester begins	Weighs ~1.1 kg; prenatal visit frequency may increase
30	Fetal position becoming observable	Cephalic presentation often established
32	Preterm window opens	Fetal movement counts typically initiated 28-32 wks
34	Lung maturity often complete	Antenatal corticosteroids beneficial if delivery <7 days likely
36	Term preterm boundary	Weekly prenatal visits begin; cervical checks typical
37	Early term	Labour may begin any day; 78% delivered by 40 weeks
38	Brain growth + descent	Fetal reflexes mature; descent into pelvis begins
39	Full term (ACOG 2013)	Optimal delivery window opens
40	EDD reached	Mean delivery date; 5% deliver on exact EDD
41	Late term	Induction often offered; 50% delivered by 41 weeks
42	Post-term (mandatory induction)	Induction mandatory per ACOG 2013

The "early term" 37 0/7 cutoff reflects ACOG 2013's recognition that babies born at 37–38 weeks have measurably higher morbidity (NICU admission, transient tachypnea, respiratory distress) than those born at 39–40 weeks. Modern practice deliberately delays elective deliveries until 39 weeks when medically possible, reducing average NICU admission rates by approximately 20%.

ACOG 2013 term pregnancy subcategories

The 2013 ACOG Committee Opinion 579 (reaffirmed 2020) refined the term classification:

ACOG 2013 Term Pregnancy Classification

Category	Gestational age	Clinical implication
Preterm	<37 0/7 weeks	NICU typically required; morbidity correlates inversely with gestational age
Early Term	37 0/7 – 38 6/7 weeks	Slightly higher morbidity than 39–40 wks; not recommended for elective induction
Full Term	39 0/7 – 40 6/7 weeks	Optimal delivery window; lowest neonatal morbidity
Late Term	41 0/7 – 41 6/7 weeks	Induction often considered; 39% delivered by 41+0 weeks
Post-Term	≥42 0/7 weeks	Mandatory induction/c-section; mortality risk rises sharply

The 2013 update reduced NICU admissions by ~20% in the United States by encouraging practitioners to delay elective inductions until 39 weeks. The "39 weeks" milestone has become a quality metric for hospital obstetric performance.

Prenatal care schedule (CDC + ACOG recommendations)

Standard prenatal care visit frequency:

Visit timing	Frequency	Content
Weeks 4–28	Monthly	Confirmation, dating, screening (NT, NIPT, quad screen), anatomy ultrasound schedule
Weeks 28–36	Bi-weekly	Growth scan, glucose screening, Tdap vaccine, fetal movement counts
Weeks 36–40+	Weekly	Cervical checks, GBS swab, position assessment, induction discussion
Post-40 weeks	2x weekly	NST (non-stress test) and BPP (biophysical profile) for fetal wellbeing

Common tests by gestational week:

8–12 wks: First-trimester combined screen (NT + serum markers)
10–12 wks: NIPT (optional; some providers order based on combined screen results)
15–22 wks: Quad test (if first-trimester screen was skipped)
18–22 wks: Anatomy ultrasound
24–28 wks: Glucose challenge test / 50g GCT (gestational diabetes screening)
28 wks: Tdap vaccine, Rhogam if Rh-negative
35–37 wks: Group B Streptococcus (GBS) culture
38+ wks: Discuss delivery plan

Ultrasound dating accuracy by gestational week

The accuracy of ultrasound-based EDD varies by gestational age at the time of scan:

Ultrasound Dating Accuracy

Gestational age at scan	Method	±Accuracy	Resets EDD threshold
7–9 weeks	CRL	±5 days	≥5 day discrepancy with LMP
9–14 weeks	CRL	±7 days	≥7 day discrepancy
14–22 weeks	BPD	±10 days	≥10 day discrepancy
22–28 weeks	BPD	±14 days	≥14 day discrepancy
>28 weeks	Late-stage biometric	±21+ days	Not typically used for redating

Source: ACOG Committee Opinion 579 (2013, reaffirmed 2020).

The first-trimester ultrasound is the most accurate dating method because fetal CRL is consistent across populations and is not affected by maternal size, ethnicity, or fetal position. Second-trimester BPD has wider variation because of differences in fetal head shape (brachycephaly, dolichocephaly), growth disturbances, and population variation. The "≥5 day discrepancy" rule (vs LMP) means if LMP-based and CRL-based EDDs differ by 5 or more days at <9 weeks, the CRL-based EDD replaces LMP-based EDD.

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