Pregnancy Calculator

Pregnancy Calculator: How Due Dates Are Estimated, What Gestational Age Means, and How to Use Milestones Wisely

1) What is an EDD and why is it an estimate?

Estimated Due Date (EDD) is the calendar day around which a pregnancy is most likely to reach 40 weeks of gestational age. “Estimate” is the key word: only a small percentage of births occur on the exact EDD. Most healthy pregnancies deliver within a window—from about 37 to 41+ weeks. EDD helps schedule routine care, screen at the right times, and plan logistics. It is not a prediction of the exact day your baby will arrive.

Why the spread? Human biology varies. Ovulation doesn’t always occur on day 14. Implantation can take 6–12 days. Some people have short cycles, others long. And growth patterns differ across pregnancies. A due date is therefore a helpful anchor for care, not a clock that you “fail” if labor begins earlier or later.

2) Four ways to date a pregnancy: LMP, Conception, IVF, Ultrasound

LMP method. The most common approach uses the first day of your last menstrual period (LMP). The classic Naegele rule adds 280 days to that date. If your cycles are consistently longer or shorter than 28 days, many clinicians adjust by the difference (cycle length − 28). Our calculator does this automatically. LMP dating works well when periods are regular and you remember the date confidently.

Conception date. If you know when fertilization occurred (for example, based on ovulation testing or a known insemination), your EDD is approximately 266 days later. This method can be more precise than LMP for irregular cycles because it does not assume ovulation on day 14.

IVF transfer. For IVF, your embryo’s age at transfer matters. The practical formula is: EDD = transfer date + (266 − embryo age). Common cases are day‑3 (+263) and day‑5 (+261). Our tool supports presets and a custom embryo age if needed. Once EDD is known, the associated “LMP” for dating purposes is just EDD − 280 days.

Ultrasound dating. Ultrasound can measure embryo or fetus size and infer gestational age. If an ultrasound on a specific date says you were, say, 12 weeks and 3 days, then the EDD is the scan date plus the remaining days to 280. Early ultrasounds (particularly in the first trimester) have the narrowest error bars and are often used to confirm—or adjust—LMP‑based dates.

3) Cycle length & irregular periods — how to adjust

With predictable cycles, adjusting is simple: if your cycles average 30 days, add two days to the naive LMP EDD; if they average 26 days, subtract two. If cycles are highly irregular, early ultrasound dating is typically preferred because it measures growth directly rather than assuming ovulation timing. Either way, the calculator lets you compare methods and pick the one your clinician recommends.

4) Gestational age vs fetal age

Gestational age (GA) counts from the start of the cycle that conceived the pregnancy—formally from the LMP (or the LMP equivalent that corresponds to your chosen method). Fetal age is roughly two weeks less and counts from fertilization. That’s why conception‑based EDD adds 266 days but LMP‑based EDD adds 280 days: they reference different day‑zeros but converge on the same 40‑week endpoint.

5) Trimester boundaries & what to expect in each

While practices vary slightly, a common framework is:

• First trimester: 0–13 weeks 6 days. Organ formation begins; early viability checks and first‑trimester screening often occur here.
• Second trimester: 14–27 weeks 6 days. Many people feel best during this time. Anatomy scan typically occurs around 18–22 weeks. Some begin to feel movement.
• Third trimester: 28 weeks onward. Growth monitoring, glucose testing window (24–28 weeks), group B strep screening (~35–37 weeks), and birth planning.

Our calculator prints date ranges for these trimesters so you can see what’s coming at a glance.

6) Key prenatal milestones & common appointment windows

• Dating/viability ultrasound: often in the first trimester to confirm location and heartbeat.
• First‑trimester screening/NT window: typically 11w0d–13w6d.
• Anatomy scan: commonly 18–22 weeks.
• Glucose screen: typically 24–28 weeks.
• Rhogam: for Rh‑negative pregnancies, usually around 28 weeks (per provider advice).
• TDap/Flu/Covid vaccines: timing per public‑health guidance—ask your clinician.
• GBS swab: around 35–37 weeks.
• “Full term”: often considered 39–40 weeks (elective deliveries before 39 weeks are usually avoided without a medical reason).

Your provider will adapt this schedule to your medical history and local guidelines. Use the milestone table as a planning aid, not a prescription.

7) Accuracy: which method is “best” and when

In general, early ultrasound dating (e.g., crown‑rump length in the first trimester) is the most precise single measurement. LMP is excellent when cycles are regular and the date is certain. Conception/IVF inputs can be very accurate because they anchor ovulation and fertilization timing. Later ultrasounds still help but have wider normal ranges. Many clinicians combine methods: use the most reliable early input to set the official EDD, then track growth against that standard.

8) How to use this calculator step‑by‑step

1. Select your dating method (LMP, Conception/IVF, or Ultrasound).
2. Enter the relevant dates and, if LMP, your average cycle length.
3. Press Calculate. The tool returns:
   • EDD (weekday + date)
   • Gestational age today (weeks + days)
   • Trimester boundaries with exact dates
   • Key milestone windows (NT, anatomy, glucose, GBS, full term)
4. Use the Copy Share Link to save or share your inputs with family, partner, or your doula.
5. Download the results or Print/Save as PDF for your records.

9) Special cases: twins, IVF nuances, late care

Twins and more. Dating formulas are the same, but care plans and timing often differ—your team will tailor appointments and delivery planning. IVF nuances. If you know your egg retrieval date or fertilization day precisely, that can anchor dating; our IVF preset accounts for embryo age at transfer. Late care. If you begin prenatal care later in pregnancy, a targeted ultrasound can help establish or confirm dating; afterwards, consistency in tracking matters more than the specific method used.

10) Frequently asked questions

Disclaimer: This calculator is for education only and does not replace medical advice. Always consult your obstetrician or midwife for personalized recommendations.