How to Use the Anomaly Scan Calculator
The Anomaly Scan Calculator determines the ideal calendar window to schedule your mid-pregnancy anatomy ultrasound:
- Choose Calculation Method — Select From Last Period (LMP) if you know the first day of your last cycle, or From Due Date (EDD) if you have an ultrasound-confirmed due date.
- Enter Your Date — Input your LMP start date or confirmed due date.
- Adjust Cycle Length — If using LMP mode, specify your average cycle length (standard: 28 days) to adjust estimated ovulation timing.
- Review Your Timeline — Instantly view your current gestational age, optimal scan window (20+0 to 22+0 weeks), and early/late scheduling boundaries.
Formula & Theory — Anomaly Scan Calculator
The Anomaly Scan Calculator applies standard obstetric dating conventions (Naegele’s rule adjusted for cycle length):
Estimated Conception Date = LMP + (Cycle Length − 14 days)
Estimated Due Date (EDD) = LMP + 280 days + (Cycle Length − 28 days)Scan date windows are derived as follows:
Earliest Scan Date (18+0 weeks) = EDD − 154 days
Optimal Start Date (20+0 weeks) = EDD − 140 days
Optimal End Date (22+0 weeks) = EDD − 126 days
Latest Scan Date (23+6 weeks) = EDD − 113 days| Milestone | Gestational Age | Purpose & Clinical Focus |
|---|---|---|
| Earliest Window | 18+0 – 19+6 Weeks | Early anatomic assessment in high-risk pregnancies |
| Optimal Window | 20+0 – 22+0 Weeks | Gold standard for comprehensive organ and cardiac screening |
| Extended Window | 22+1 – 23+6 Weeks | Follow-up scans or sub-optimal acoustic visualization |
Major Anatomical Structures Assessed
During the 20-week anomaly scan, accredited sonographers systematically examine:
- Central Nervous System: Lateral ventricles, cerebellum, cisterna magna, and intact spine contour.
- Cardiovascular Anatomy: Four-chamber heart view, left and right ventricular outflow tracts, aortic arch, and rhythm.
- Gastrointestinal & Urinary Systems: Stomach bubble, both kidneys, renal pelvis dimensions, urinary bladder, and abdominal wall closure.
- Musculoskeletal System & Face: Upper and lower limbs, five digits per extremity, femur length, nasal bone, and intact upper lip.
- Biometry & Supporting Structures: Biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), femur length (FL), placental localization, and amniotic fluid volume.
Clinical Utility of the Anomaly Scan Calculator
- Timely Appointment Booking: Specialized prenatal centers often book weeks in advance; calculating your window early ensures an appointment in the optimal 20–22 week timeframe.
- Clinical Alignment: Matches international guidelines (ISUOG, ACOG, RCOG) for structural fetal anomaly screening.
- Preparation & Peace of Mind: Helps expectant parents understand the scope and clinical milestones of mid-pregnancy ultrasound screening.