SN6
SHORT NOTE
5M
Describe the basis of pregnancy tests. Add a note on the causes of false results.
Basis of the Test
Pregnancy tests detect human chorionic gonadotrophin (hCG), a glycoprotein secreted by the syncytiotrophoblast from the time of implantation.
- Composed of an α subunit, shared with LH, FSH and TSH, and a β subunit that is unique
- Modern tests are directed at the β subunit, which is why they do not cross-react with LH
HCG Levels
| Time | Serum hCG |
|---|---|
| Implantation (day 6–8) | First detectable |
| Doubling time in early pregnancy | 48–72 hours |
| Peak | 8–10 weeks (about 100,000 IU/L) |
| After 12 weeks | Falls to a plateau |
| After delivery | Undetectable in 2 weeks |
Types of Test
| Test | Detects from | Sensitivity |
|---|---|---|
| Serum β-hCG | 8–9 days after ovulation | Most sensitive; 1–2 IU/L; quantitative |
| Urine immunoassay (home kit) | Around the missed period | 20–25 IU/L; qualitative |
| Transvaginal ultrasound | 4–5 weeks | Gestational sac visible when hCG is about 1500 IU/L |
- Best done on an early morning sample, which is most concentrated
Causes of False Results
| False positive | False negative |
|---|---|
| Hydatidiform mole, choriocarcinoma | Testing too early |
| HCG-secreting tumours (lung, ovary) | Dilute urine |
| Recent pregnancy, abortion or delivery | Expired or faulty kit |
| HCG injections for ovulation induction | Hook effect — very high hCG saturates the antibody |
| Proteinuria, haematuria | Ectopic pregnancy with low hCG |
Applied Aspects
- Serial hCG is the key investigation in early pregnancy problems — a rise slower than 66% in 48 hours suggests ectopic pregnancy or a failing intrauterine pregnancy
- Very high hCG → suspect molar pregnancy or multiple pregnancy; hCG is also the tumour marker used to follow gestational trophoblastic disease
- HCG is used therapeutically to trigger ovulation in infertility treatment, because it mimics the LH surge
- Hyperemesis gravidarum and the mild thyroid stimulation of early pregnancy are both attributed to high hCG, which weakly cross-reacts with the TSH receptor
All questions in Reproductive System
- Q1Describe the ovarian and uterine changes of the menstrual cycle. Explain its hormonal control. ★
- Q2Define spermatogenesis. Describe its stages, the cells of the testis and the factors controlling it. ★
- Q3Describe the hormones of the placenta and the maternal physiological changes during pregnancy. Add a note on parturition. ★
- Q4Describe the actions and regulation of testosterone. Add a note on puberty in the male. ★
- Q5Describe the physiology of lactation. Explain why milk secretion begins only after delivery. ★
- SN1Describe the mechanism of ovulation and enumerate its indicators. ★
- SN2Classify the methods of contraception. Describe the mechanism of the combined oral contraceptive pill. ★
- SN3Define menopause. Describe the hormonal changes and clinical features. ★
- SN4Describe semen analysis with its normal values. Add a note on the composition of semen. ★
- SN5Describe the changes at puberty in boys and girls and the mechanism of its onset.
- SN6Describe the basis of pregnancy tests. Add a note on the causes of false results.
- SN7Define infertility. Enumerate its causes and outline the investigations. ★