How Pregnancy Metrics calculates dates, weeks, ovulation timing, weight-gain and test-timing references

Pregnancy Record is a versioned browser data model and orchestration layer. It calls the existing pregnancy-date, due-date, conception, timeline, and IVF pure functions; it does not reimplement or change their medical arithmetic.

Each dating record preserves its raw input, controlled record type, calculated EDD and pregnancy-calendar start, formula version, timestamps, and provenance. A user explicitly selects the record used as the current Pregnancy Metrics timeline basis. A current clinical EDD can only be marked on a user-entered recorded-EDD record and remains a separate concept.

Changing the timeline basis appends previous and new record identifiers to basis history. It recalculates current display context but does not rewrite raw dating records or dated weight measurements. Differences are descriptive calendar-day comparisons; no source is scored, averaged, or automatically declared correct.

An ultrasound entry accepts only exam date plus reported completed weeks and additional days and reuses dated-pregnancy-age arithmetic. It does not infer gestational age from CRL, BPD, other biometry, or an image, and it does not automatically apply a redating threshold.

Standard 28-day LMP path:
Pregnancy-calendar start = recorded LMP

Cycle-adjusted LMP path:
Pregnancy-calendar start = adjusted EDD − 280 days
Equivalent: pregnancy-calendar start = recorded LMP + (cycle length − 28 days)

From a due date in the pregnancy record:
Implied pregnancy start = recorded due date − 280 days
Total pregnancy days = selected date − implied pregnancy start

Pregnancy weeks on a reference date = reference date − pregnancy-calendar start. The recorded LMP remains the user's actual input. In a cycle-adjusted path, the adjusted pregnancy-calendar start is a derived dating reference, not another actual menstrual period; the adjustment does not identify the exact ovulation date. The established clinical EDD remains the priority for care.

Completed weeks = whole-number division by 7. Additional days = remainder from 0 through 6. For example, 8 weeks and 4 days means 8 completed weeks plus 4 additional days.

The result follows the record entered. It does not establish or revise a clinical due date.

Example: January 1, 2024 to March 1, 2024 = 60 calendar days = 8 weeks, 4 days

Read the Pregnancy Dating and Timeline Guide for completed-week notation, reference dates, trimester boundaries, and term timing.

The Trying & Testing Timeline starts with one supported record. An LMP and cycle can show one coherent estimated cycle plus its expected-period testing reference; the other records show only the testing reference or window they support.

• Last period and cycle: cycle day 1 is the recorded LMP; estimated ovulation cycle day is usual cycle length − 14; the six-day fertile window ends on that estimated date; the expected period is LMP + cycle length.
• Expected period: the supplied expected period is the reference date.
• Well-supported ovulation: ovulation date + 12 through +15 calendar days in a 28-day context.
• Current DPO: browser-local today + (12 − DPO) through today + (15 − DPO).
• Most recent relevant sex date: date + 21 calendar days when the next period is unknown.

USER-ENTERED, DERIVED, and APPROXIMATE labels keep source records, direct arithmetic, and biological estimates distinct. These are calendar references, not pregnancy-probability calculations or guarantees. Timing depends on ovulation, implantation, hCG production, urine concentration, test sensitivity, and the instructions for the test used. Fertility hCG medicines, recent pregnancy, and uncertain ovulation timing require the relevant clinical boundary rather than an additional calculator mode.

The cycle path uses formula version ovulation-fertile-window-v1. The LMP (first day of the last menstrual period) is cycle day 1. Estimated ovulation cycle day = usual cycle length − 14, and the date is the LMP plus cycle day − 1 calendar days.

The primary fertile window is the estimated ovulation day and the five previous days, matching the ASRM six-day calendar definition. The two days before estimated ovulation are shown as an estimated higher-fertility part of that window; this is not an individual pregnancy probability. The estimated next period is LMP + usual cycle length.

The supported model range is 21–35 whole days. It estimates one entered cycle, does not auto-roll to a new cycle, and does not forecast multiple cycles. It cannot confirm ovulation, diagnose fertility, identify safe days, or serve as contraception.

LH tests, cervical mucus, and basal body temperature are explained as context but are not algorithm inputs. Calendar estimates become less dependable when cycle timing varies substantially, the LMP is uncertain, periods are absent, or fertility or hormonal treatment changes applicability.

Home-test paths use formula version pregnancy-test-timing-v1. Combining the interface does not create a new formula version or change either engine.

This model uses pre-pregnancy BMI to select the published singleton and twin weight-gain ranges from the National Academies framework used by ACOG and CDC. It is an educational guideline reference, not a personal target, diagnosis, or nutrition prescription.

BMI = pre-pregnancy weight in kilograms ÷ height in meters squared. The unrounded BMI determines the category: 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 the pregnancy gain ranges; the tool does not create separate obesity classes.

Singleton total ranges are 12.5–18 kg (28–40 lb) for underweight, 11.5–16 kg (25–35 lb) for normal BMI, 7–11.5 kg (15–25 lb) for overweight, and 5–9 kg (11–20 lb) for obesity. Published second- and third-trimester rates are shown as guideline context, not as an exact cumulative week-by-week formula. The first-trimester assumption is approximately 0.5–2.0 kg (1.1–4.4 lb), and this tool does not classify first-trimester progress from a single weight.

The Pregnancy Metrics weight-gain model, version pregnancy-weight-gain-v2, keeps the published BMI-specific total ranges and second- and third-trimester rate context visible without inventing a cumulative week-by-week band. The CDC provides official weekly trackers for people who need that comparison; this page does not digitize or reconstruct one without authoritative numerical source data.

Current change = current weight − pre-pregnancy weight. The displayed approximate end-weight range is the pre-pregnancy baseline plus the published total gain range. Negative current change is valid input; it is not interpreted as a diagnosis. Calculations use kilograms internally, with 1 inch = 2.54 cm and 1 lb = 0.45359237 kg; display values are rounded to one decimal place after classification and range calculations.

Twin total ranges are 22.7–28.1 kg (50–62 lb) for underweight, 17–25 kg (37–54 lb) for normal BMI, 14–23 kg (31–50 lb) for overweight, and 11–19 kg (25–42 lb) for obesity. The underweight twin range is the CDC-published exception to the broader IOM/NAM framework. The tool does not derive a twin weekly trajectory. For triplets or more, no numeric range or trajectory is presented because a standard range is not established; discuss the record with the care team.

Technical input limits (height 100–250 cm, weight 20–400 kg, and gestational age 0w0d–42w6d) prevent obvious entry errors; they are not medical thresholds. The optional tracker is off by default and stores only a canonical metric baseline and gestational-age/weight points in localStorage after explicit opt-in. It stores no calendar dates, identifiers, or free-text notes.

Pure calendar functions live in src/lib/pregnancy-dates.ts. All arithmetic uses date-only UTC conversion so that browser timezone differences cannot shift a result by one calendar day. Leap years and cross-year calculations are covered by tests.

Calculator inputs are processed in the browser. No calculation API or account is required.

Formula version: pregnancy-dates-v2

The maximum 45-week, 0-day span is a supported calculator range, not a normal or official clinical range. Where that range applies, 45 weeks, 1 day and later is rejected.

The IVF transfer-date input supports January 20, 1000 through March 8, 9999. This technical range keeps the implied pregnancy-calendar start, estimated due date, and maximum 45-week, 0-day reference date within a four-digit YYYY-MM-DD year. It is not a medical eligibility range.

The site does not display a pregnancy-progress percentage because it is not a measure of maturity, development, or safety.

If you are not sure which calculator fits the information you have, use the calculator selection guide. The editorial and methodology review explains the content maintenance process. Read the privacy notice for how browser calculation and routine hosting request data are handled, and the medical disclaimer for the educational-use limits of these tools.