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Antenatal Calculator 2.0

EDD, gestation, ultrasound re-dating & PHCC obstetric ultrasound booking windows

📅 1. Dates from LMP

Enter the first day of the last menstrual period. Leave blank if the LMP is unknown or unreliable and date from the scan alone.

First day of last menstrual period (LMP)

🧬 2. Re-date by ultrasound (optional)

Enter a first-trimester scan to confirm or revise the dates. Use CRL (crown–rump length) or the gestational age the sonographer reported.

Date of ultrasound scan

Dating method

Crown–rump length (CRL)

Valid range 10–84 mm (≈7+1 to 14+0 weeks). Above 84 mm, date by BPD/HC instead.

📊 Results

Enter an LMP date and/or an ultrasound scan, then select Calculate dates.

Re-dating follows ACOG / AIUM / SMFM Committee Opinion 700: the LMP date is kept unless the scan differs by more than the threshold for that gestation. CRL is converted with the Robinson & Fleming (1975) formula.

📋 Minimum Required Information — all OBS US orders

Field	Documentation requirement
1. LMP	Select the most reliable date.
2. Requested Start Date	Select the 1st or 2nd start date of the booking window based on current GA on date of order. If current GA is already within the booking window: select the next working date for routine & urgent.
3. Clinical History	1. Add current GA at date of order, AND 2. Add the booking window (end date of the recommended scan period).
4. Reason for Exam	For viability US or early/repeat growth US: mandatory documentation of risk or clinical indication. For standard ANC scan, document: NT / Anatomy / Growth US.
5. Pregnant	Select “Yes”.

Source: PHCC OBS ultrasound ordering standards. Verify against current local guidance.

Educational aid — apply clinical judgement and current PHCC guidance.