How Pregnancy Metrics calculations work

Pregnancy dating methods

The Due Date Calculator starts with one supported source. With a reliable last menstrual period (LMP), estimated due date (EDD) = LMP + 280 days for a 28-day model. A supported average cycle length adjusts that estimate by (cycle length − 28 days). The 21–35 whole-day input range limits this model; it is not a diagnostic threshold or proof of regular ovulation. Uncertain LMP or substantially variable cycles weaken a simple calendar estimate.

With a well-supported ovulation date or documented fertilization date, EDD = that date + 266 days. Intercourse is not a documented fertilization date. A dated pregnancy-age record gives an implied pregnancy-calendar start = record date − recorded pregnancy days, then EDD = that start + 280 days. The tool does not interpret ultrasound measurements or decide clinical redating.

The Gestational Age Calculator counts days from the selected dating anchor to a chosen reference date. Its LMP mode starts at the recorded LMP; its established-EDD mode uses an implied start of recorded EDD − 280 days. Separately, a cycle-adjusted Due Date Calculator result uses adjusted EDD − 280 days, equivalently recorded LMP + (cycle length − 28 days), as its pregnancy-calendar start. The recorded LMP remains the actual input; the adjusted start is a derived dating reference, not another menstrual period or an exact ovulation date. Completed weeks are whole groups of seven days; additional days are the remainder from 0 through 6. January 1 to March 1, 2024 is 60 days, or 8 weeks, 4 days.

The Conception Date Calculator reports an around-date estimate: recorded EDD − 266 days; or, from LMP, the derived pregnancy-calendar start + 14 days; or, from dated pregnancy weeks, record date − recorded pregnancy days + 14 days. A 28-day LMP of January 1, 2024 yields around January 15 and a related EDD of October 7. This cannot establish exact fertilization, intercourse, implantation, or parentage, and it does not give a universal uncertainty window.

The IVF Due Date Calculator uses the actual transfer date and a clinic-confirmed total embryo developmental age. Day-5 transfer EDD = transfer date + 261 days; Day-3 transfer EDD = transfer date + 263 days. Gestational age on transfer day is 2 weeks, 5 days or 2 weeks, 3 days respectively. Fresh and frozen transfers use the actual transfer date. For frozen-thawed embryos, the clinic-confirmed total age includes development before freezing and after thawing or warming; do not reconstruct it from retrieval or freezing dates. If Day 3 or Day 5 is not confirmed, use the clinic's EDD rather than guessing. A March 1, 2024 transfer gives November 17 for Day 5 or November 19 for Day 3.

For a less formula-focused explanation, read Pregnancy Dating Explained or How IVF Due Dates Are Calculated.

Ovulation and fertile-window estimate

The Ovulation Calculator starts with LMP and a usual whole-day cycle length of 21–35 days. LMP is cycle day 1; estimated ovulation is cycle day (cycle length − 14). The estimated six-day fertile window is that day and the five preceding days. The two days before estimated ovulation are shown as a higher-fertility part of the window, not an individual pregnancy probability. The next expected period is LMP + cycle length.

This is a one-cycle calendar estimate, not confirmation of ovulation, a fertility diagnosis, or a contraceptive method. Cycle variability, uncertain dates, absent periods, and fertility or hormonal treatment can reduce its usefulness.

Pregnancy-test timing

The Pregnancy Test Calculator uses one of five references: an entered expected-period date; LMP + usual cycle length; a well-supported ovulation date + 12 through 15 days; current days past ovulation (DPO), placed against the browser's local today and the same 12–15 DPO window; or the most recent relevant sex date + 21 days when the next period is unknown. The LMP path estimates the expected period, not ovulation or implantation.

These are general home-test timing references, not pregnancy probabilities or guarantees. Ovulation and implantation timing, hCG production, urine concentration, test sensitivity, and product instructions affect interpretation. hCG-containing fertility medication, a recent pregnancy, and IVF or IUI clinic schedules need separate interpretation; the calculator does not create treatment-specific dates or confirm pregnancy.

Pregnancy weight-gain references

The Pregnancy Weight Gain Calculator uses pre-pregnancy BMI = weight in kilograms ÷ height in meters squared. Classification uses the unrounded BMI: underweight below 18.5, normal from 18.5 to below 25, overweight from 25 to below 30, and obesity at 30 or above. BMI 40 or above remains in the same obesity category for these ranges.

Published singleton total-gain ranges are 12.5–18 kg (28–40 lb), 11.5–16 kg (25–35 lb), 7–11.5 kg (15–25 lb), and 5–9 kg (11–20 lb), respectively. Twin total ranges are 22.7–28.1 kg (50–62 lb), 16.8–24.5 kg (37–54 lb), 14.1–22.7 kg (31–50 lb), and 11.3–19.1 kg (25–42 lb). The underweight twin range is the CDC-published exception to the broader National Academies framework. No standard numeric range is assigned for triplets or more.

For singletons only, Pregnancy Metrics derives an optional week-specific reference from 13w0d to 40w0d by linearly interpolating between the approximate 0.5–2.0 kg first-trimester reference at 13w0d and the BMI-specific published total-range endpoints at 40w0d. At each endpoint, reference gain = start + ((gestational days − 91) ÷ (280 − 91)) × (term endpoint − start). Published second- and third-trimester rates are shown as context, not used as another cumulative formula. This derived trajectory is not an official CDC week-by-week curve, a validated percentile, diagnosis, or personal target. It is not extrapolated after 40w0d or used to classify first-trimester progress or a current value as on or off track. Twins have total ranges but no derived weekly trajectory.

Optional current change = current weight − pre-pregnancy weight; approximate end-weight range = baseline weight + the published total-gain range. Calculations use kilograms internally (1 inch = 2.54 cm; 1 lb = 0.45359237 kg), and displayed values are rounded after classification. A negative current change is valid input, not a diagnosis. These population ranges do not prescribe diet, weight loss, or treatment; discuss individual circumstances with a care team.

Clinical records and ultrasound

First-trimester ultrasound through 13 weeks, 6 days is the most accurate ultrasound method to establish or confirm gestational age. IVF pregnancy dating uses embryo age and transfer date when those records are available. Once a care team selects and documents an EDD, later changes should be uncommon and clinically documented. Use the current EDD in the pregnancy record rather than averaging several dates. This site cannot interpret crown–rump length or other ultrasound measurements, judge scan quality, or apply clinical redating criteria.

Weeks, trimesters, and term timing

Pregnancy age uses completed weeks plus additional days: 36 weeks, 3 days means 36 completed weeks and 3 days. The first trimester runs through 13w6d, the second from 14w0d through 27w6d, and the third starts at 28w0d.

From 37w0d, early term is 37w0d–38w6d; full term is 39w0d–40w6d; late term is 41w0d–41w6d; postterm begins at 42w0d. These are timing categories, not diagnoses, safety ratings, or instructions about delivery.

Method limits

The calculations do not establish an individual clinical due date, confirm exact conception or implantation, assess pregnancy location or viability, predict IVF success or delivery day, or determine treatment or urgent-care needs. The Medical Disclaimer explains these medical-use boundaries; Editorial and Methodology Review explains who checks the content and when it is revisited.

Official references

Each source below supports the stated part of the methods; citation does not imply that the organization approves or endorses Pregnancy Metrics.

• ACOG — Methods for Estimating the Due DateLast-period dating, early ultrasound, IVF dating by embryo age and transfer date, and documenting a clinical due date.
• ACOG — How Your Fetus Grows During PregnancyPregnancy-week and trimester conventions, and the estimated rather than guaranteed nature of a due date.
• Merck Manual Professional — Evaluation of the Obstetric PatientThe last-period +280-day and conception +266-day estimated-delivery conventions.
• ASRM — The International Glossary on Infertility and Fertility Care, 2025ART gestational age and cumulative embryo developmental age for frozen-thawed transfers.
• ACOG — Definition of Term PregnancyEarly-, full-, late-, and postterm timing boundaries.
• ACOG — Fertility Awareness-Based Methods of Family PlanningThe typical 28-day cycle, approximate ovulation timing, and sperm survival context.
• ASRM — Optimizing Natural Fertility: A Committee OpinionFertile-window timing and the limits of predicting an individual's ovulation from a calendar.
• ACOG — Weight Gain During PregnancyBMI-based singleton and twin total ranges and the need for individualized clinical judgment.
• ACOG patient guidance — How Much Weight Should I Gain During Pregnancy?First-trimester variability and the contextual nature of weight-gain guidance.
• CDC — Weight Gain During PregnancyBMI-specific singleton and twin ranges, including the underweight twin exception and the triplets-plus boundary.
• National Academies — Weight Gain During Pregnancy: Reexamining the GuidelinesThe BMI-based guideline framework and published second- and third-trimester rate context.
• NICE — Maternal and child nutrition: nutrition and weight managementThe non-prescriptive boundary around intentional pregnancy weight loss, diet, and treatment.