Question:easy
Lady using OCP since 5 months. Amenorrhea since last 6 weeks. Which is best to calculate gestational age in this case?
Lady using OCP since 5 months. Amenorrhea since last 6 weeks. Which is best to calculate gestational age in this case?
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When LMP is unreliable due to OCP use, which first-trimester ultrasound parameter gives the most accurate gestational age?
Updated On: Jun 23, 2026
- 280 days from LMP
- 256 days from LMP
- CRL by USG
- Abdominal girth
Show Solution
The Correct Option is C
Solution and Explanation
Concept tested: Gestational age estimation when LMP is unreliable.
This patient has been on OCPs for 5 months. OCPs suppress the hypothalamic-pituitary-ovarian axis, preventing ovulation. Therefore, the LMP on OCPs does not represent an ovulatory cycle and cannot be used for accurate gestational age calculation using Naegele's rule ($\text{EDD} = \text{LMP} + 9 \text{ months} + 7 \text{ days}$).
Why CRL by USG is the answer:
CRL (Crown-Rump Length) is the gold standard for dating in the first trimester (6--13 weeks). It is a direct ultrasound measurement of embryonic length and is independent of menstrual history.
Accuracy of CRL: $\pm 3$--5 days in early first trimester.
After 13 weeks, preferred parameters shift to:
- Biparietal diameter (BPD)
- Head circumference (HC)
- Femur length (FL)
LMP-based methods (280 days / 256 days / Naegele's rule) are all unreliable here.
Abdominal girth varies with body habitus and is not a gestational age marker.
\[\boxed{\text{CRL by USG}}\]
This patient has been on OCPs for 5 months. OCPs suppress the hypothalamic-pituitary-ovarian axis, preventing ovulation. Therefore, the LMP on OCPs does not represent an ovulatory cycle and cannot be used for accurate gestational age calculation using Naegele's rule ($\text{EDD} = \text{LMP} + 9 \text{ months} + 7 \text{ days}$).
Why CRL by USG is the answer:
CRL (Crown-Rump Length) is the gold standard for dating in the first trimester (6--13 weeks). It is a direct ultrasound measurement of embryonic length and is independent of menstrual history.
Accuracy of CRL: $\pm 3$--5 days in early first trimester.
After 13 weeks, preferred parameters shift to:
- Biparietal diameter (BPD)
- Head circumference (HC)
- Femur length (FL)
LMP-based methods (280 days / 256 days / Naegele's rule) are all unreliable here.
Abdominal girth varies with body habitus and is not a gestational age marker.
\[\boxed{\text{CRL by USG}}\]
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