[{"data":1,"prerenderedAt":418},["ShallowReactive",2],{"/en/guides/pregnancy/what-is-gestational-age":3},{"id":4,"title":5,"body":6,"category":390,"description":391,"extension":392,"faq":393,"meta":409,"navigation":412,"path":413,"publishedAt":410,"relatedCalculator":363,"seo":414,"stem":415,"updatedAt":416,"__hash__":417},"guides_en/en/guides/pregnancy/what-is-gestational-age.md","What Is Gestational Age? Definition, Calculation and How It Differs from Fetal Age",{"type":7,"value":8,"toc":375},"minimark",[9,13,28,31,36,46,49,53,119,126,132,136,139,152,156,161,164,169,173,180,183,187,255,258,262,331,335,366,369],[10,11,5],"h1",{"id":12},"what-is-gestational-age-definition-calculation-and-how-it-differs-from-fetal-age",[14,15,16],"blockquote",{},[17,18,19,23,24,27],"p",{},[20,21,22],"strong",{},"Source:"," NHS, American College of Obstetricians and Gynecologists (ACOG) | ",[20,25,26],{},"Last reviewed:"," April 2026",[17,29,30],{},"Gestational age is the standard way of measuring how far along a pregnancy is. It is used by all healthcare providers — midwives, obstetricians, sonographers, and neonatologists — to schedule antenatal care, assess fetal development, and determine when a baby is at term.",[32,33,35],"h2",{"id":34},"definition","Definition",[17,37,38,41,42,45],{},[20,39,40],{},"Gestational age"," is the age of a pregnancy counted from the ",[20,43,44],{},"first day of the last menstrual period (LMP)",", expressed in weeks and days.",[17,47,48],{},"This is the number your healthcare provider refers to when they say \"you are 12 weeks pregnant\" or \"the baby was born at 34 weeks.\" It is not the same as how old the embryo or fetus is from the moment of fertilisation.",[32,50,52],{"id":51},"gestational-age-vs-fetal-age","Gestational Age vs. Fetal Age",[54,55,56,71],"table",{},[57,58,59],"thead",{},[60,61,62,65,68],"tr",{},[63,64],"th",{},[63,66,67],{},"Gestational Age",[63,69,70],{},"Fetal (Embryonic) Age",[72,73,74,86,97,108],"tbody",{},[60,75,76,80,83],{},[77,78,79],"td",{},"Counted from",[77,81,82],{},"First day of LMP",[77,84,85],{},"Date of conception/fertilisation",[60,87,88,91,94],{},[77,89,90],{},"Difference",[77,92,93],{},"~2 weeks longer",[77,95,96],{},"~2 weeks shorter",[60,98,99,102,105],{},[77,100,101],{},"Full term",[77,103,104],{},"40 weeks",[77,106,107],{},"~38 weeks",[60,109,110,113,116],{},[77,111,112],{},"Used by",[77,114,115],{},"Healthcare providers (standard)",[77,117,118],{},"Embryology, IVF settings",[17,120,121,122,125],{},"Because ovulation (and therefore conception) typically occurs around day 14 of a 28-day cycle, gestational age is approximately ",[20,123,124],{},"2 weeks more"," than fetal age.",[17,127,128,131],{},[20,129,130],{},"Example:"," A fetus at 38 weeks of fetal development has a gestational age of approximately 40 weeks.",[32,133,135],{"id":134},"why-the-lmp-is-used-as-the-starting-point","Why the LMP Is Used as the Starting Point",[17,137,138],{},"The LMP is used as the start date because:",[140,141,142,146,149],"ul",{},[143,144,145],"li",{},"It is a known, recordable date the pregnant person can usually recall",[143,147,148],{},"The exact date of conception is almost always uncertain without specialised monitoring",[143,150,151],{},"It creates a consistent, standardised reference point used globally in obstetric practice",[32,153,155],{"id":154},"how-gestational-age-is-calculated","How Gestational Age Is Calculated",[157,158,160],"h3",{"id":159},"method-1-from-lmp-naegeles-rule","Method 1: From LMP (Naegele's Rule)",[17,162,163],{},"LMP + 280 days = estimated due date (40 weeks gestational age)",[17,165,166,168],{},[20,167,130],{}," LMP of 1 January 2026 → due date approximately 8 October 2026",[157,170,172],{"id":171},"method-2-ultrasound","Method 2: Ultrasound",[17,174,175,176,179],{},"An ultrasound between 8 and 14 weeks measures the ",[20,177,178],{},"crown-rump length (CRL)"," — the distance from the top of the head to the base of the spine. This is compared to reference tables to estimate gestational age.",[17,181,182],{},"First trimester ultrasound dating is accurate to within 5–7 days and is the gold standard when LMP is unknown or uncertain.",[32,184,186],{"id":185},"gestational-age-classifications","Gestational Age Classifications",[54,188,189,198],{},[57,190,191],{},[60,192,193,196],{},[63,194,195],{},"Classification",[63,197,67],{},[72,199,200,208,216,224,232,239,247],{},[60,201,202,205],{},[77,203,204],{},"Extremely preterm",[77,206,207],{},"Before 28 weeks",[60,209,210,213],{},[77,211,212],{},"Very preterm",[77,214,215],{},"28–31+6 weeks",[60,217,218,221],{},[77,219,220],{},"Moderate to late preterm",[77,222,223],{},"32–36+6 weeks",[60,225,226,229],{},[77,227,228],{},"Early term",[77,230,231],{},"37–38+6 weeks",[60,233,234,236],{},[77,235,101],{},[77,237,238],{},"39–40+6 weeks",[60,240,241,244],{},[77,242,243],{},"Late term",[77,245,246],{},"41–41+6 weeks",[60,248,249,252],{},[77,250,251],{},"Post-term",[77,253,254],{},"42 weeks and beyond",[17,256,257],{},"A baby born before 37 weeks is preterm. Most guidelines recommend delivery monitoring from 41 weeks, with induction typically offered by 42 weeks to reduce post-term risks.",[32,259,261],{"id":260},"how-gestational-age-structures-antenatal-care","How Gestational Age Structures Antenatal Care",[54,263,264,273],{},[57,265,266],{},[60,267,268,270],{},[63,269,67],{},[63,271,272],{},"Standard Check or Scan",[72,274,275,283,291,299,307,315,323],{},[60,276,277,280],{},[77,278,279],{},"8–14 weeks",[77,281,282],{},"Dating scan (confirms gestational age)",[60,284,285,288],{},[77,286,287],{},"11–14 weeks",[77,289,290],{},"Combined first trimester screening",[60,292,293,296],{},[77,294,295],{},"~16 weeks",[77,297,298],{},"Midwife appointment, blood results review",[60,300,301,304],{},[77,302,303],{},"18–21 weeks",[77,305,306],{},"Anomaly (mid-pregnancy) scan",[60,308,309,312],{},[77,310,311],{},"24–36 weeks",[77,313,314],{},"Regular appointments every 2–4 weeks",[60,316,317,320],{},[77,318,319],{},"38–40 weeks",[77,321,322],{},"Final check-ups",[60,324,325,328],{},[77,326,327],{},"41–42 weeks",[77,329,330],{},"Post-dates monitoring and induction discussion",[32,332,334],{"id":333},"related-guides","Related Guides",[140,336,337,345,352,359],{},[143,338,339,344],{},[340,341,343],"a",{"href":342},"/guides/pregnancy/how-to-calculate-your-due-date","How to Calculate Your Due Date"," — LMP, conception, and ultrasound dating methods",[143,346,347,351],{},[340,348,350],{"href":349},"/guides/pregnancy/lmp-vs-conception-date","LMP vs Conception Date"," — when the two methods give different answers",[143,353,354,358],{},[340,355,357],{"href":356},"/guides/pregnancy/pregnancy-trimesters-explained","Pregnancy Trimesters Explained"," — key milestones at each stage",[143,360,361,365],{},[340,362,364],{"href":363},"/pregnancy-due-date-calculator","Pregnancy Due Date Calculator"," — calculate your EDD instantly",[367,368],"hr",{},[17,370,371],{},[372,373,374],"em",{},"This guide is for general information only, based on NHS and ACOG guidance. Gestational age confirmed by ultrasound takes precedence over LMP-based estimates.",{"title":376,"searchDepth":377,"depth":377,"links":378},"",2,[379,380,381,382,387,388,389],{"id":34,"depth":377,"text":35},{"id":51,"depth":377,"text":52},{"id":134,"depth":377,"text":135},{"id":154,"depth":377,"text":155,"children":383},[384,386],{"id":159,"depth":385,"text":160},3,{"id":171,"depth":385,"text":172},{"id":185,"depth":377,"text":186},{"id":260,"depth":377,"text":261},{"id":333,"depth":377,"text":334},"pregnancy","Plain-language explanation of gestational age — how it is measured from the last menstrual period (LMP), why it differs from fetal age by approximately 2 weeks, and how it guides antenatal care.","md",[394,397,400,403,406],{"question":395,"answer":396},"What is gestational age?","Gestational age is the age of a pregnancy measured from the first day of the last menstrual period (LMP), expressed in weeks and days. A full-term pregnancy is 39–40 weeks gestational age. It is the standard measure used in antenatal care worldwide.",{"question":398,"answer":399},"How is gestational age different from fetal age?","Gestational age counts from the LMP, which is approximately 2 weeks before conception. Fetal age (embryonic/conception age) counts from fertilisation. A baby at 40 weeks gestational age is approximately 38 weeks from fertilisation. Medical references almost always use gestational age.",{"question":401,"answer":402},"How is gestational age measured?","It is first estimated from the LMP date using Naegele's rule (LMP + 280 days = due date). It is then confirmed or revised by ultrasound — the crown-rump length (CRL) at 8–14 weeks is the most accurate measurement for determining gestational age.",{"question":404,"answer":405},"What does it mean to be born at 37 weeks gestational age?","A baby born at 37 weeks gestational age is early term. Full term is 39–40+6 weeks. Preterm is before 37 weeks; late term is 41 weeks; post-term is 42 weeks or beyond.",{"question":407,"answer":408},"Why does gestational age start 2 weeks before conception?","Because it starts from the LMP, which is a reliably known date, whereas the exact date of conception is often uncertain. The 2-week offset is a practical convention that makes pregnancy dating consistent globally.",{"lastVerified":410,"sourceFamily":411},"2026-04-02","NHS, ACOG",true,"/en/guides/pregnancy/what-is-gestational-age",{"title":5,"description":391},"en/guides/pregnancy/what-is-gestational-age",null,"VpmtJrmLK6Vk5gwlr9mAcUajC2p73akaDYLVgP-JZqQ",1775557776569]