Pregnancy Week Calculator
Reviewed by Hamza Gujjar, Algorithm Architect & Software Developer
Last updated: August 20, 2026"How Far Along Am I" Has an Exact Answer
Gestational age is simply elapsed time counted in completed weeks and days, but the reference point people have on hand varies — some know their last period date, others only have a due date from a clinician. This calculator accepts either and converts it to the same result: your current week and day, your trimester, and how many days remain until your due date.
The Formula
Weeks Pregnant = Days Pregnant ÷ 7 (remainder = extra days)
Due Date = Last Period Start Date + 280 days
When you start from a due date instead, the calculator simply works backward — Last Period Start Date = Due Date − 280 days — and then runs the same forward calculation from there.
Trimester Boundaries Used Here
| Completed weeks | Trimester |
|---|---|
| 0 to 12 | 1st Trimester |
| 13 to 26 | 2nd Trimester |
| 27 onward | 3rd Trimester |
These cutoffs are conventions, not biological thresholds — other sources sometimes draw the line a week either way. These are the specific boundaries this calculator applies.
Why Two Starting Points
- From last period — the default if you're tracking your own cycle and haven't yet had a dating scan.
- From due date — useful once a clinician has given you a due date, particularly if it was adjusted by ultrasound and no longer matches simple LMP arithmetic.
Both modes converge on the same output format, so switching between them mid-pregnancy — for instance after a dating scan revises your due date — is straightforward.
How to Use This Calculator
- Select a mode: From Last Period Date or From Due Date.
- Enter the relevant date — your Last Period Start Date, or your Due Date.
- Select Calculate for your current week and day, trimester, due date, and days remaining.
Related Calculations
For the original due-date calculation with a cycle-length adjustment, see the Due Date Calculator. To check your weight gain against a pacing curve for your current week, use the Pregnancy Weight Gain Calculator.
Principles of Obstetric Gestational Age: Pregnancy Weeks
A pregnancy week calculator determines the current gestational age of a pregnancy in weeks and days from the first day of the Last Menstrual Period (LMP). In obstetrics and prenatal care, accurate gestational dating is critical for scheduling prenatal screening tests, monitoring fetal growth, and determining safe delivery timing.
The Fundamental Gestational Age Formulas
Gestational Age (Weeks + Days) = Total Days / 7 (Weeks) remainder (Days)
Estimated Due Date (Naegele Rule): EDD = LMP + 280 Days (40 Weeks + 0 Days)
Remaining Weeks = 40 Weeks - Current Gestational Weeks
Trimester Classification and Key Prenatal Milestones
| Trimester | Gestational Weeks | Key Prenatal Milestones and Screenings |
|---|---|---|
| First Trimester | Weeks 1 to 13 | Confirmation ultrasound (6 to 8 weeks); First trimester screening / NIPT (10 to 13 weeks); Nuchal translucency (NT) scan |
| Second Trimester | Weeks 14 to 27 | Anatomy scan ultrasound (18 to 22 weeks); Glucose tolerance test (24 to 28 weeks); Fetal viability milestone (24 weeks) |
| Third Trimester | Weeks 28 to 40+ | Group B Strep screening (35 to 37 weeks); Non-stress tests (NST); Bishop score assessment near term |
Crown-Rump Length (CRL) Dating in Early Pregnancy
During the first trimester (6 to 13 weeks), ultrasound measurement of the fetal Crown-Rump Length (CRL) provides the most accurate gestational age assessment, with an accuracy margin of plus or minus 3 to 5 days. If CRL dating differs from LMP dating by more than 7 days, the ultrasound-adjusted due date replaces the LMP-calculated date.
Fetal Viability and the Periviable Period
The gestational age threshold for potential fetal survival outside the womb is critically important in perinatology:
- 22 to 23 Weeks: Extreme prematurity; neonatal survival rates 10% to 40% with aggressive NICU intervention. Family counseling required.
- 24 Weeks (Viability Milestone): Survival rates 40% to 70%; active neonatal resuscitation is standard of care.
- 28 Weeks: Survival rates exceed 90% in level III NICUs.
- 34 Weeks (Late Preterm): Survival > 99%; short NICU stay for feeding and temperature support.
Antenatal Corticosteroid Administration by Gestational Week
When preterm delivery is anticipated between 24 and 34 weeks gestational age, obstetric guidelines mandate administration of Betamethasone 12 mg IM × 2 doses (24 hours apart):
Antenatal corticosteroids accelerate fetal lung maturity surfactant production, reducing neonatal respiratory distress syndrome (RDS), intraventricular hemorrhage (IVH), and necrotizing enterocolitis (NEC) by over 50%.
Term Delivery Classification
| Classification | Gestational Weeks | Clinical Significance |
|---|---|---|
| Early Term | 37w0d to 38w6d | Slightly increased neonatal respiratory risk; elective delivery discouraged before 39 weeks |
| Full Term | 39w0d to 40w6d | Optimal neonatal outcomes; ideal delivery window |
| Late Term | 41w0d to 41w6d | Increased monitoring; induction of labor often offered |
| Post-Term | ≥ 42w0d | Elevated placental insufficiency risk; delivery recommended |
Group B Streptococcus (GBS) Screening at 35 to 37 Weeks
Between 35 and 37 weeks gestational age, all pregnant women undergo Vaginal-Rectal Group B Streptococcus (GBS) Culture Screening:
GBS-positive mothers receive intrapartum IV Penicillin G (5 million units loading dose, then 2.5 to 3.0 million units every 4 hours until delivery) to prevent neonatal early-onset GBS sepsis, meningitis, and pneumonia.
Bishop Score for Cervical Ripeness Assessment
When labor induction is planned near term (39 to 41 weeks), the Bishop Score evaluates five cervical characteristics (dilation, effacement, station, consistency, and position):
- Bishop Score ≥ 8: Favorable cervix; high probability of successful induction with oxytocin alone.
- Bishop Score < 6: Unfavorable cervix; cervical ripening agents (misoprostol, dinoprostone, or Foley balloon catheter) are recommended before oxytocin augmentation.
Non-Stress Test (NST) Monitoring in Third Trimester
Beginning at 32 to 36 weeks in high-risk pregnancies (gestational diabetes, preeclampsia, advanced maternal age), obstetricians perform Weekly Non-Stress Tests (NST):
A reactive NST demonstrates two or more fetal heart rate accelerations of at least 15 beats per minute lasting at least 15 seconds within a 20-minute monitoring window, confirming adequate fetal oxygenation and reassuring placental function. Non-reactive NSTs prompt immediate biophysical profile (BPP) ultrasound evaluation.
Importance of Accurate Gestational Dating
Precise gestational age determination affects virtually every aspect of prenatal care: scheduling appropriate screening tests, interpreting laboratory and ultrasound results, counseling regarding fetal viability at the threshold of extreme prematurity, and making informed decisions about optimal delivery timing to maximize both maternal and neonatal outcomes.
Reliable gestational age tracking empowers expectant parents and their healthcare providers to navigate every stage of pregnancy with informed confidence and optimal clinical outcomes.