Every dating method in this guide is listed with its ACOG or CDC source on the editorial standards page.
Most of this site's calculators trace back to the same starting point, your last menstrual period, and then branch depending on what you actually want to know: a due date, a current week, a conception window, or whether an early lab result or weight gain figure is tracking a published range. This page is the map. It is informational; your provider's own dating and guidance take precedence over anything here.
The LMP-dating group: due date, pregnancy, trimester, gestational age
The Due Date Calculator, Pregnancy Calculator, Trimester Calculator and Gestational Age Calculator all run Naegele's rule: LMP plus 280 days, cycle-adjusted. The mechanics are explained in the due date calculation guide, the week-counting logic in how many weeks pregnant am I, and the term categories in the full-term guide. Where an ultrasound has been done, ACOG's redating thresholds, detailed on the editorial standards page, determine whether the scan or the LMP math should be trusted.
The conception-window group: ovulation, conception, conception date
The Ovulation Calculator, Conception Calculator and Conception Date Calculator work from the same day-14-of-28 assumption, just applied in the other direction: instead of adding 280 days forward from LMP, they estimate roughly 266 days back from a due date, or about 14 days forward from LMP. The conception date guide covers both directions and explains why the honest output is a several-day window rather than one date.
Early signals: hCG doubling and implantation
The hCG Doubling Time Calculator and Implantation Calculator sit slightly apart from the date-math group above; they describe biological patterns rather than calendar arithmetic. hCG commonly doubles every 48 to 72 hours in very early pregnancy at low starting levels, a pattern documented in the clinical hCG literature and summarized on the editorial standards page, which is why a single hCG number tells a clinician far less than the trend across two draws does.
Tracking through pregnancy: period and weight gain
The Period Calculator is a general cycle-tracking tool useful before and alongside a pregnancy. The Pregnancy Weight Gain Calculator runs the 2009 National Academies (formerly Institute of Medicine) ranges, which set a different recommended total based on your pre-pregnancy BMI category, with a separate range for a twin pregnancy. These are population guidelines; your provider tailors an actual target to your own health.
Using these tools without over-trusting them
Every calculator here treats its input as reliable and returns a matching estimate; none of them can tell that a cycle was unusual, that a scan has already redated the pregnancy, or that a specific health condition changes the picture. Cross-check a result against what your provider has told you, not the other way around. The full list of sources behind every method is on the editorial standards page, dated and linked.
All calculators on this site
- Due Date Calculator - LMP or conception date, cycle-adjusted.
- Pregnancy Calculator - Current week and trimester.
- Ovulation Calculator - Fertile window and likely ovulation date.
- Conception Calculator - Estimated conception date from a due date.
- Conception Date Calculator - Estimated conception window from LMP.
- hCG Doubling Time Calculator - Doubling time between two hCG results.
- Pregnancy Weight Gain Calculator - Recommended range by pre-pregnancy BMI.
- Trimester Calculator - Current trimester and days remaining.
- Period Calculator - Cycle and next-period tracking.
- Implantation Calculator - Likely implantation window.
- Gestational Age Calculator - Gestational age from LMP or ultrasound date.
Frequently asked questions
Which calculator should I start with?
The Due Date Calculator if you know your LMP or an ultrasound date; the Ovulation Calculator if you are tracking cycles before a pregnancy.
Do these tools replace an ultrasound?
No. A first-trimester ultrasound is more accurate than LMP-based math per ACOG, and it should be trusted over these calculators when the two disagree beyond the threshold on the editorial standards page.
Why do the conception tools give a range instead of one date?
Because sperm can survive several days and the exact moment of fertilization is not detectable after the fact; a several-day window is the honest answer, explained in the conception date guide.
Are these calculators a substitute for prenatal care?
No. They are planning estimates built from published methods, meant to sharpen the questions you bring to an appointment, not to replace one.