Question

Lady using OCP since 5 months. Amenorrhea since last 6 weeks. Which is best to calculate gestational age in this case?

• A. 280 days from LMP
• B. 256 days from LMP
• C. CRL by USG
• D. Abdominal girth

Hint:

When LMP is unreliable due to OCP use, which first-trimester ultrasound parameter gives the most accurate gestational age?

Answer 1

The Correct Option is C

Step 1: Identify the problem with LMP-based dating. This patient has been on OCPs (oral contraceptive pills) for 5 months. OCPs suppress ovulation and alter the menstrual cycle. The last menstrual period (LMP) while on OCPs may not correspond to a true ovulatory cycle, making LMP-based gestational age calculation unreliable.

Step 2: Understand CRL (Crown-Rump Length). CRL is measured on ultrasound as the largest dimension of the embryo, excluding the yolk sac and extremities. It is the most accurate parameter for estimating gestational age between 6 and 13 weeks (first trimester).

• CRL by USG gives an accurate gestational age independent of menstrual history
• After 13 weeks, other parameters such as biparietal diameter (BPD), head circumference (HC), and femur length (FL) are used

Step 3: Eliminate other options.

• 280 days from LMP (Naegele's rule): Unreliable when LMP is on OCPs
• 256 days from LMP: Used in some ethnic populations but still based on LMP
• Abdominal girth: Not a reliable indicator of gestational age

Conclusion: CRL by USG is the best method for gestational age calculation when LMP is unreliable due to OCP use.