Gestational Age Calculator
How many weeks and days, adjusted for cycle length.
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You are counted as pregnant before you conceived
Gestational age is counted from the first day of your last menstrual period, not from conception. Since ovulation happens around two weeks into a cycle, this means you are considered two weeks pregnant at the moment of conception, and four weeks pregnant about two weeks after it — usually around the time a test first turns positive.
The convention exists because the LMP is a date most people can report, while conception almost never is: ovulation is not directly observable and sperm survive up to five days. It does assume a regular 28-day cycle with ovulation on day 14, which is why cycle length matters here. A 35-day cycle typically ovulates around day 21, shifting both gestational age and the due date by roughly a week.
Ultrasound dating is more accurate, particularly in the first trimester, where crown-rump length predicts gestational age within a few days. If ultrasound and LMP dating disagree by more than about a week, antenatal care will normally redate the pregnancy to the scan. Trimesters run roughly weeks 1-13, 14-27 and 28 to birth, though the exact boundaries vary slightly between sources.
Pregnancy Milestones by Gestational Week & Trimester
Clinical pregnancy dating tracks fetal milestones in completed weeks and days rather than calendar months. Here is what to expect across each key gestational window.
| Trimester | Gestational Weeks | Key Clinical Milestones & Fetal Development |
|---|---|---|
| First Trimester | Weeks 1 – 13 | Weeks 1–3: Menstrual cycle, ovulation, fertilization, and blastocyst formation. Week 4: Uterine implantation; hCG hormone rises, detectable on home tests. Weeks 6–8: Fetal pole and cardiac activity visible on early transvaginal ultrasound. Weeks 11–13: Crown-rump length (CRL) measurement dating; Nuchal Translucency (NT) screening. |
| Second Trimester | Weeks 14 – 27 | Weeks 16–20: Quickening (first noticeable fetal movements); quad screening. Weeks 18–22: Level II Anatomy Ultrasound scan evaluates all fetal organs and placental position. Week 24: Fetal viability threshold; lungs produce preliminary surfactant. Weeks 24–28: Maternal gestational diabetes screening test (glucose tolerance). |
| Third Trimester | Weeks 28 – 40+ | Week 28: Rapid brain development and rapid weight gain begins. Weeks 35–37: Group B Strep (GBS) screening swab; baby settles into cephalic (head-down) presentation. Weeks 37–38: Early Term. Organs are fully formed. Weeks 39–40: Full Term. Estimated Due Date (EDD). |
Naegele's Rule & Cycle Adjustment Formula
Standard obstetrical dating relies on Naegele's rule, which calculates the estimated date of delivery (EDD) based on a 280-day gestational period from the first day of the last normal menstrual period (LMP):
Because the follicular phase of a menstrual cycle can fluctuate while the luteal phase remains relatively constant at 14 days, women with longer cycles (e.g. 35 days) ovulate later (approx. Day 21). Our calculator dynamically factors this deviation into your true gestational age.
Medical disclaimer. This calculator is for general information and education only. It is not medical advice, not a diagnosis, and not a substitute for professional care. Dating from a last period assumes a regular cycle. Your antenatal team’s ultrasound dating takes precedence over any calculation, and irregular cycles make LMP dating substantially less reliable.Speak to a doctor or another qualified clinician about any health decision.Last reviewed .
Frequently asked questions
Why is pregnancy counted from the last period?
Because the first day of the last period is a date most people can report, while conception rarely is — ovulation is not observable and sperm survive several days. The convention adds roughly two weeks before conception actually occurred.
How does cycle length change the calculation?
Ovulation timing shifts with cycle length while the luteal phase stays near 14 days. A 35-day cycle typically ovulates around day 21 rather than day 14, which moves gestational age and the due date about a week later.
Is ultrasound dating more accurate?
Yes, particularly in the first trimester, where crown-rump length predicts gestational age within a few days. If scan dating and LMP dating differ by more than about a week, antenatal care normally redates to the scan.
When do the trimesters start and end?
Approximately weeks 1-13 for the first, 14-27 for the second, and 28 until birth for the third. Exact boundaries vary slightly between sources, and nothing clinical changes on a specific day.
What does 8w 3d mean?
Eight completed weeks and three days of gestational age. Pregnancy is described in completed weeks plus days rather than in months, because the weekly scale is what clinical milestones are tied to.