In a normally fertile woman, how many days after conception will a urine pregnancy test and a quantitative serum β‑hCG test become positive?

Timing of Positive Pregnancy Tests After Conception

Urine pregnancy tests typically become positive 3-4 days after implantation (which occurs 6-12 days post-fertilization), meaning approximately 9-16 days after conception, while serum β-hCG tests can detect pregnancy 1-2 days earlier due to higher sensitivity. 1, 2

Understanding the Biological Timeline

The detection of pregnancy depends on a cascade of events that must occur after conception:

• Implantation occurs 6-12 days after fertilization, with significant variability between individuals 1, 2
• hCG production begins only after implantation occurs, not at the moment of conception 1
• This biological variability means there is no single "days after conception" answer that applies universally

Urine Pregnancy Test Detection

Most FDA-approved urine pregnancy tests have a sensitivity threshold of 20-25 mIU/mL 3, 4, 5

Practical Detection Timeline:

• 3-4 days after implantation: Most sensitive urine tests (20-25 mIU/mL) can detect pregnancy 1, 2
• By the first day of missed period: Only 90% of pregnancies are detectable, because 10% have not yet implanted 6
• By 7 days after missed period: 97-98% of pregnancies are detectable 1, 2
• An additional 11 days past expected menses may be needed to detect 100% of pregnancies with standard qualitative tests 1, 4

Critical Caveat About Commercial Tests:

Despite marketing claims, actual clinical performance varies dramatically between brands 7, 8. Research shows:

• Only 1 of 18 tested brands had sufficient sensitivity (12.5 mIU/mL) to detect 95% of pregnancies at the time of missed menses 8
• Tests with 100 mIU/mL sensitivity would detect only approximately 16% of pregnancies at the time of missed period 8
• Many over-the-counter tests claiming ">99% accuracy" actually detect far fewer early pregnancies than advertised 7, 9

Serum β-hCG Test Detection

Serum pregnancy tests are more sensitive than urine tests and can detect pregnancy earlier 1, 5

Key Advantages:

• hCG first appears in maternal blood 6-8 days following fertilization 2
• Serum tests have lower detection thresholds than the 20-25 mIU/mL standard for urine 5
• Serum testing should be considered when clinical suspicion is high but urine test is negative 1

Practical Clinical Algorithm

For Testing 3+ Weeks After Unprotected Intercourse:

• Test immediately with urine pregnancy test 1
• By this timepoint, 97-98% of pregnancies will be detectable 1

For Testing <3 Weeks After Unprotected Intercourse:

• Consider serum β-hCG if early detection is clinically important 1, 5
• If urine test is negative but pregnancy remains possible, repeat testing in 3-4 days or consider serum testing 1, 5

If Initial Test is Negative:

• Repeat urine testing in 3-4 days if intercourse was recent 1
• Repeat in 1 week if symptoms persist 1
• A negative test 1 week after missed period virtually guarantees non-pregnancy 2

Important Pitfalls to Avoid

• Do not rely on "first day of missed period" testing as definitive—10% of pregnancies have not implanted by this time 6
• Pregnancy tests cannot detect pregnancy from very recent sexual intercourse because implantation has not yet occurred 4
• Later implantation produces slower hCG rise, which delays detection 4
• The timing of ovulation varies widely (days 9-20 of a 28-day cycle), affecting when conception actually occurs 3, 5