Pregnancy Calculator
Find your due date, see how many weeks pregnant you are, track every trimester, and follow your baby's full 40-week journey.
The first day your last period began — the most common starting point for pregnancy dating.
Medically Reviewed
Dr. Emily Carter, MD — Board-Certified OB-GYN
Dating methods, week-by-week milestones, and references on this page are cross-checked against ACOG clinical guidance, NIH/NICHD research, CDC reproductive health data, and WHO pregnancy standards. Read our full Editorial Policy.
How Is a Pregnancy Due Date Calculated?
The most widely used method is Naegele's Rule, which calculates the estimated due date (EDD) as the first day of your last menstrual period (LMP) plus 280 days (40 weeks). This assumes a regular 28-day cycle with ovulation on day 14.
Due Date = LMP + 280 days
or equivalently: LMP + 9 months + 7 days
Since most women don't know their exact ovulation or conception date, the LMP method provides a consistent reference point. Gestational age counts from the LMP, not from conception — which is why you're considered "2 weeks pregnant" even before the egg is fertilized.
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LMP Method
Most common. Add 280 days to the first day of your last period. Works best with regular 28-day cycles.
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Ultrasound Dating
Gold standard in early pregnancy. Crown-rump length at 6–14 weeks is the most accurate measurement.
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IVF Transfer
Date is known precisely. 5-day blastocyst transfer: add 261 days. 3-day embryo: add 263 days.
The Three Trimesters Explained
First Trimester
Weeks 1–12- • Most critical period of organ formation — all major organs develop
- • Morning sickness, fatigue, and breast tenderness are common
- • Miscarriage risk is highest in weeks 1–9, dropping sharply after week 12
- • First prenatal visit, blood tests, and nuchal translucency scan happen here
Second Trimester
Weeks 13–27- • Often called the "golden trimester" — energy returns and nausea subsides
- • Baby's movements (quickening) become noticeable around weeks 18–22
- • Anatomy ultrasound at 18–20 weeks checks all organ systems
- • Glucose challenge test at 24 weeks screens for gestational diabetes
Third Trimester
Weeks 28–40- • Baby gains 70% of birth weight during this trimester
- • Braxton Hicks contractions and back pain become more common
- • Weekly visits begin at week 36; Group B Strep test performed
- • Baby considered full term at 37 weeks; due date is week 40
What is LMP and Why Does it Matter?
LMP (Last Menstrual Period) refers to the first day of your most recent menstrual period before becoming pregnant. It's the standard starting point for pregnancy dating because it's a known, observable event — unlike ovulation or conception, which are harder to pinpoint precisely.
Gestational age — the "how many weeks pregnant" number — is counted from LMP, not from conception. This means at the moment of conception (roughly 2 weeks after LMP), you're already considered 2 weeks pregnant. At a positive pregnancy test (around 4 weeks), you've technically been pregnant for 4 weeks by this measure.
IVF Pregnancy Timeline
IVF pregnancies are dated differently because fertilization and embryo development happen in a lab rather than in the body. The transfer date is the key reference point:
5-Day Blastocyst Transfer (most common)
LMP equivalent = Transfer Date − 19 days
Due Date = Transfer Date + 261 days
The embryo spent 5 days in the lab, plus ovulation typically occurs 14 days after LMP, so: LMP = transfer − 14 − 5 = transfer − 19.
3-Day Embryo Transfer
LMP equivalent = Transfer Date − 17 days
Due Date = Transfer Date + 263 days
3-day embryos are less developed; they spend more time growing inside the uterus after transfer.
How Many Weeks Pregnant Am I?
Your current pregnancy week is your gestational age — the number of complete weeks that have passed since the first day of your last menstrual period (LMP). At a positive home pregnancy test you're typically already 4 weeks pregnant. At your first OB-GYN visit (usually 8–10 weeks), the doctor will confirm your gestational age with a dating ultrasound.
Gestational age
Counted from your LMP — the global obstetric standard. When your OB-GYN says "you are 12 weeks pregnant," they mean gestational age. The 40-week pregnancy timeline is measured this way.
Fetal age (embryonic age)
Counted from conception — roughly 2 weeks less than gestational age. A 10-week gestational age = ~8 weeks fetal age. Gestational age is what doctors use; fetal age is rarely cited clinically.
Why count from the LMP? The LMP is a known calendar date, while ovulation and conception are difficult to identify precisely. This convention, codified as Naegele's Rule in 1812, gives every clinician — and every pregnancy app — a consistent framework. Use our Age Calculator to count exact days between two dates if you want to verify week math manually.
Pregnancy Due Date: Ultrasound vs LMP
Three methods are commonly used to estimate the due date. ACOG (American College of Obstetricians and Gynecologists) ranks first-trimester ultrasound as the gold standard, with LMP and conception-date methods serving as backups.
| Method | Accuracy | Best Time to Use | Common Use Cases |
|---|---|---|---|
| First-trimester ultrasound (crown-rump length) | ±3–5 days | Weeks 6–14 | Gold standard; mandatory if LMP is uncertain or cycles are irregular. |
| Second-trimester ultrasound | ±7–10 days | Weeks 14–22 | Used when first-trimester scan wasn't done; less precise than crown-rump. |
| LMP (Naegele's Rule) | ±7 days | Any time | Default when LMP is known and cycles are regular 28-day cycles. |
| Conception date | ±1–3 days | If date is known | Best for IVF or carefully tracked ovulation; rarely usable in spontaneous pregnancy. |
| IVF transfer date | ±0 days | Always for IVF | Most precise — exact embryo age is known from the lab. |
ACOG guidance: if the first-trimester ultrasound EDD differs from the LMP-based EDD by more than 5–7 days, the ultrasound date should be used. After 22 weeks, an established EDD is rarely changed.
Pregnancy Symptoms by Week
Symptoms vary widely person to person, but research published in major OB-GYN journals shows clear week-by-week patterns. Here's a general guide.
Week 4
Missed period (often the first sign). Mild cramping as the embryo implants. Light spotting (implantation bleeding) in some women. Breast tenderness begins. hCG rises sharply.
Week 6
Nausea and morning sickness peak — often worst in weeks 6–9. Fatigue intensifies due to progesterone. Frequent urination starts. Sense of smell heightens. Heartbeat detectable on transvaginal ultrasound.
Week 8
Food aversions and cravings become noticeable. Mood swings from hormonal shifts. Slight uterine growth — clothes may feel tighter. Breast size visibly increases. Acne or skin changes possible.
Week 12
First trimester ends. Nausea typically eases. Energy starts returning. Slight 'baby bump' may appear (especially in second pregnancies). Nuchal translucency scan often performed this week.
Week 20
Mid-pregnancy 'golden trimester' — energy is at its highest. Anatomy ultrasound performed (weeks 18–22). Baby's movements (quickening) felt by most women. Pregnancy bump clearly visible.
Week 28
Third trimester begins. Heartburn, back pain, and swelling become common. Braxton Hicks (practice contractions) start. Glucose challenge test screens for gestational diabetes around this point. Baby's kicks are strong.
Week 36
Baby drops lower into the pelvis ('lightening'). Increased pelvic pressure. More frequent urination returns. Group B Strep test performed. Weekly OB-GYN visits begin. Energy decreases as delivery approaches.
Symptoms are general guidance only. Any concerning symptom — heavy bleeding, severe pain, fever, severe vomiting, or reduced fetal movement — warrants immediate contact with your OB-GYN or midwife.
What Happens If You Deliver Early or Late?
ACOG defines five delivery windows around the estimated due date. Each carries different clinical considerations.
Preterm (< 37 weeks 0 days)
Babies born before 37 weeks are considered premature. The CDC reports about 10% of US births are preterm. Risks include underdeveloped lungs, low birth weight, jaundice, and feeding difficulties. NICU care is typically required before 34 weeks.
Early term (37 weeks 0 days – 38 weeks 6 days)
Often called 'early term.' Most babies do well, but research shows slightly higher rates of breathing and feeding issues compared to full-term babies. Elective inductions before 39 weeks are no longer routinely recommended by ACOG without medical indication.
Full term (39 weeks 0 days – 40 weeks 6 days)
Optimal window for delivery. Lungs, brain, and other organs are fully mature. About 60% of all births fall in this window. The due date itself (40 weeks 0 days) is just a median — only ~5% of babies are born on it.
Late term (41 weeks 0 days – 41 weeks 6 days)
Most pregnancies in this window remain healthy, but monitoring becomes more frequent. ACOG recommends discussing induction options between 41 weeks 0 days and 42 weeks 0 days due to gradually increasing perinatal risks.
Post-term (≥ 42 weeks 0 days)
Pregnancies extending beyond 42 weeks (post-term) carry higher risks: placental aging, meconium aspiration, and umbilical cord complications. Induction is typically recommended by 42 weeks. Only 0.5% of US births are post-term.
Pregnancy Milestones Week by Week
Major developmental milestones from implantation to full term. This timeline shows when key events typically occur — your individual experience may shift by a few days either way.
Week 4 — Implantation
The fertilized egg implants in the uterine lining. hCG begins rising — detectable by home pregnancy tests.
Week 6 — First heartbeat
Baby's heart begins beating, detectable on transvaginal ultrasound. Neural tube starts forming.
Week 8 — All organs begin forming
Brain, spinal cord, heart, kidneys, and limbs are all developing. Embryo is now technically a fetus.
Week 12 — Organs largely developed
End of first trimester. Major organ systems formed. Miscarriage risk drops sharply. Fingernails and external genitals begin forming.
Week 16 — Hearing develops
Baby can hear sounds. Movement becomes more coordinated. Most parents-to-be feel comfortable announcing the pregnancy.
Week 20 — Anatomy scan
Mid-pregnancy ultrasound checks every major organ system. Fetal sex often confirmed. You're halfway through pregnancy.
Week 24 — Viability milestone
Lungs begin producing surfactant. With NICU support, babies born at 24 weeks have a chance of survival (though significant complications remain likely).
Week 28 — Third trimester begins
Baby gains 70% of birth weight from here onward. Brain growth accelerates. Eyes open for the first time.
Week 32 — Sleep cycles begin
Baby has distinct sleep and wake cycles. Movements are noticeable from outside. Lungs continue maturing.
Week 37 — Early term reached
Baby is considered early term. Most organs are ready for life outside the womb. Lung maturity continues to refine until week 39.
Week 39 — Full term
ACOG-defined full-term begins. Optimal week for delivery. Brain and lungs are fully mature.
Week 40 — Due date
Median due date. Only ~5% of babies are born on this exact day — most arrive between weeks 37 and 42.
For weight-gain tracking during pregnancy, see our BMI Calculator; for nutritional planning, our Calorie Calculator.
Medical Disclaimer: This pregnancy calculator is for educational purposes only and is not medical advice. Due dates are approximations — only about 5% of babies are born on their exact EDD. Individual factors including cycle length, ovulation timing, fetal growth, and underlying conditions affect accuracy. Always consult your obstetrician, midwife, or other qualified healthcare provider for personalized medical guidance and official pregnancy dating.
Frequently Asked Questions
Authors & Medical Review
Written By
SamCalculator Editorial Team
Women's health and pregnancy writers producing evidence-based content from ACOG, NICHD, CDC, and WHO sources. Read more on our About page.
Medically Reviewed By
Dr. Emily Carter, MD
Board-certified Obstetrician-Gynecologist (OB-GYN) with clinical focus on prenatal care, pregnancy dating, and maternal-fetal medicine. Reviews all clinical content on this page.
Last medically reviewed: May 11, 2026 · Last updated: May 11, 2026
Scientific References
- ACOG Committee Opinion No. 700 — Methods for Estimating the Due Date.
- ACOG — How Your Fetus Grows During Pregnancy (week-by-week development).
- NIH / NICHD — Pregnancy: condition information, stages, and care.
- CDC — Preterm Birth statistics and clinical definitions.
- WHO — Preterm Birth Fact Sheet.
- Jukic AM, et al. (2013). Length of human pregnancy and contributors to its natural variation. Human Reproduction.
- ACOG — Special tests for monitoring fetal well-being.
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