What methods are available for checking pregnancy?

Methods for Checking Pregnancy

Pregnancy can be confirmed through urine or blood testing for human chorionic gonadotropin (hCG), with most urine tests yielding positive results 3-4 days after implantation and 98% positive by the time of the expected period. 1, 2

Primary Detection Methods

Urine Pregnancy Tests

• Home urine pregnancy tests detect hCG using monoclonal antibodies to the beta-subunit of hCG, virtually eliminating cross-reaction with luteinizing hormone (LH). 2
• Most current pregnancy test kits have a sensitivity of 25 IU/L, allowing detection 3-4 days after implantation (approximately 6-8 days following fertilization). 2
• By 7 days after the missed period, 98% of pregnant women will test positive on urine tests. 2
• A negative urine test result 1 week after the missed period virtually guarantees the woman is not pregnant. 2
• The First Response manual and digital tests demonstrate 97% detection of pregnancy on the day of missed menstrual bleeding, with a sensitivity of 5.5 mIU/mL for the mixture of hCG forms present in early pregnancy. 3
• Other brands (EPT and ClearBlue) show lower detection rates (54-67%) on the day of missed menses despite similar claims, with sensitivity of 22 mIU/mL. 3

Blood Pregnancy Tests

• Serum beta-hCG testing may be performed for pregnancy confirmation or monitoring, particularly when clinical suspicion exists despite negative urine testing. 4
• Blood tests can detect hCG earlier than urine tests and provide quantitative measurements useful for monitoring pregnancy viability or diagnosing ectopic pregnancy. 1
• Pregnancy tests performed on blood or urine yield positive results before the first missed menses with normal intrauterine pregnancy. 1

Ultrasound Confirmation

• Ultrasound detection of pregnancy is not as sensitive as hCG measurement for early pregnancy detection. 2
• Dating ultrasound using crown-rump length measurement in the first trimester provides accurate gestational age estimation to within 7 days. 1
• Ultrasound measurement of biparietal diameter (BPD) in the second trimester estimates gestational age to within 10 days. 1

Important Technical Considerations

hCG Forms and Detection

• Hyperglycosylated hCG (H-hCG) is the principal hCG immunoreactivity in early pregnancy urine, comprising 61% and 50% of total immunoreactivity in the 4th and 5th completed weeks of pregnancy, respectively. 5
• Nine of 15 home pregnancy test devices (60%) demonstrate poorer detection limits for H-hCG compared to regular hCG, potentially affecting early detection accuracy. 5
• Beta-subunit antibodies react with both intact hCG (major component in pregnancy serum) and beta-core fragments (major form in urine). 2

Timing and Accuracy

• hCG from the implanting blastocyst first appears in maternal blood around 6-8 days following fertilization, with levels rising rapidly to peak at 7-10 weeks. 2
• Claims of pregnancy detection 8 days prior to menstruation are unrealistic despite marketing assertions. 6
• False positive results due to interfering materials are extremely unlikely with current generation test kits. 2

Clinical Application Algorithm

When to Perform Pregnancy Testing

• Pregnancy testing should be considered with missed menses (unless using etonogestrel implants or depot medroxyprogesterone acetate), irregular bleeding, new onset pelvic pain, enlarged uterus or adnexal mass on examination, or before institution of new medications with potential adverse effects on pregnancy. 1
• New onset of irregular bleeding after prolonged amenorrhea while using hormonal contraception warrants pregnancy testing. 1

Gestational Age Assessment

• The first day of the last menstrual period is the most common method for assessing gestational age, though ultrasound provides more accurate estimation. 1, 7
• If gestational age changes by 2 or more weeks after ultrasound examination, test results must be reinterpreted. 1
• A pelvic examination may be necessary to assess gestational age if the woman is uncertain about her last menstrual period date. 7

Common Pitfalls

• Failing to recognize that most qualitative pregnancy tests require an additional 11 days past expected menses to detect 100% of pregnancies can result in false negatives. 7
• Not all home pregnancy tests perform equally—sensitivity varies from 6.3-50 IU/L for regular hCG, contradicting universal claims of >99% detection on the first day of missed menses. 5
• Healthcare professionals and women should be aware that some claims made for home pregnancy tests are inconsistent, and common definitions and testing criteria are urgently needed. 6
• Overlooking the possibility of ectopic pregnancy when symptoms suggest it can lead to severe complications. 7
• Small amounts of hCG exist in blood and urine of non-pregnant subjects, which can occasionally cause confusion in interpretation. 2