Is a transvaginal ultrasound (u/s) used to establish gestational age in a patient at approximately 2 weeks gestation who has recently taken Plan B (levonorgestrel)?

Transvaginal Ultrasound at 2 Weeks Gestation

No, transvaginal ultrasound is not used to establish gestational age at 2 weeks gestation because no gestational structures are visible at this early stage, and the patient is not yet pregnant by standard obstetric dating.

Understanding Gestational Age Terminology

• Gestational age is calculated from the first day of the last menstrual period (LMP), not from conception 1, 2
• At "2 weeks" gestational age by standard obstetric dating, ovulation has just occurred or is about to occur—conception has not yet happened 1
• A gestational sac cannot be visualized until approximately 4.5-5 weeks gestational age (32 days from LMP) at the earliest 1, 2

Earliest Detection Thresholds by Transvaginal Ultrasound

When structures become visible:

• Gestational sac: First visible at 32 days gestational age (approximately 4.5 weeks) when β-hCG reaches 1000-1100 mIU/mL 1, 2
• Yolk sac: Consistently visible between 36-40 days gestational age when β-hCG reaches 5800-7200 mIU/mL 1, 2
• Embryo with cardiac activity: Consistently visible after 40-46 days gestational age when β-hCG exceeds 10,800-24,000 mIU/mL 1, 2

Clinical Context: Plan B (Levonorgestrel)

• Plan B prevents or delays ovulation and is taken as emergency contraception after unprotected intercourse [@general medical knowledge@]
• If Plan B successfully prevents pregnancy, there will be no gestational structures to visualize at any point [@general medical knowledge@]
• If Plan B fails and pregnancy occurs, the earliest detection would still follow the thresholds above—no earlier than 4.5-5 weeks gestational age 1, 2

When Transvaginal Ultrasound IS Appropriate for Dating

Optimal timing for gestational age assessment:

• First trimester dating: Crown-rump length measurement between 5-12 weeks gestational age provides the most accurate dating (±8.4 days) 3
• Second trimester dating: Composite measurements including biparietal diameter at 14-22 weeks are accurate within 10 days 4
• Transvaginal ultrasound allows measurement of crown-rump length in very early gestation (as early as 5-6 weeks) when transabdominal approaches fail 3, 5

Common Pitfall to Avoid

• Do not order ultrasound at "2 weeks" gestational age expecting to see anything—you are ordering imaging before pregnancy has even occurred by standard dating conventions 1, 2
• The earliest clinically useful transvaginal ultrasound for confirming intrauterine pregnancy is at 5-6 weeks gestational age (approximately 3-4 weeks after conception) 1, 5, 2