What is the most important investigation to perform before prescribing contraception to a patient of reproductive age presenting with abnormal uterine bleeding (AUB) for six months?

Most Important Investigation Before Prescribing Contraception in AUB

A pregnancy test (β-hCG) is the single most important investigation that must be performed before prescribing contraception to any reproductive-age woman presenting with abnormal uterine bleeding. 1, 2, 3

Mandatory First Step: Exclude Pregnancy

• Pregnancy testing is mandatory in all reproductive-age women with AUB, even in perimenopausal women, before initiating any contraceptive therapy. 1, 3
• This is non-negotiable regardless of the patient's reported sexual history or perceived likelihood of pregnancy. 2
• Failure to exclude pregnancy before prescribing hormonal contraception represents a critical clinical error that could mask complications of pregnancy or expose a developing fetus to hormonal agents. 3

Why Not the Other Options First?

TSH (Option B)

• TSH should be checked as part of the standard diagnostic workup for ovulatory dysfunction, but it is not the most important before prescribing contraception. 1, 3
• Thyroid dysfunction can cause AUB, but identifying it is not time-sensitive in the same way pregnancy exclusion is. 4

Ultrasound (Option C)

• Combined transabdominal and transvaginal ultrasound with Doppler is the most appropriate initial imaging study for identifying structural causes (polyps, adenomyosis, leiomyomas, endometrial hyperplasia/malignancy). 1, 2, 3
• However, ultrasound comes after pregnancy testing in the diagnostic algorithm. 3
• Ultrasound is particularly important in this case given the 6-month duration of bleeding, but pregnancy must be excluded first. 5

Endometrial Biopsy (Option D)

• Endometrial biopsy is indicated when risk factors for endometrial cancer are present (obesity, diabetes, hypertension, unopposed estrogen exposure, tamoxifen use, Lynch syndrome), when medical management has failed, or when ultrasound shows endometrial thickness ≥4 mm with persistent bleeding. 1, 3
• This patient's 6-month duration of AUB may warrant endometrial sampling if risk factors are present, but pregnancy testing takes absolute priority. 1

Complete Algorithmic Approach for This Patient

1. Perform pregnancy test immediately 1, 2, 3
2. Conduct speculum examination to exclude cervical or vaginal sources of bleeding 1, 3
3. Perform bimanual examination to assess uterine size, contour, mobility, and adnexal masses 1
4. Order transvaginal ultrasound (combined with transabdominal if needed) to identify structural causes given the 6-month duration 1, 3, 5
5. Check TSH and prolactin levels for ovulatory dysfunction 1, 3
6. Consider endometrial biopsy if the patient has risk factors for endometrial cancer or if ultrasound shows concerning findings 1, 3

Critical Pitfall to Avoid

Never skip pregnancy testing in reproductive-age women, even those who are perimenopausal or report no recent sexual activity. 1 Starting hormonal contraception without excluding pregnancy can have serious medicolegal and clinical consequences, including masking ectopic pregnancy or exposing an early pregnancy to exogenous hormones.