Pregnancy Test in the Menstrual Cycle

The Menstrual Cycle - Foundation

• The LH surge is a reliable predictor of ovulation, with onset occurring 16-58 hours before ovulation for 90% of women
• Cycle length variation (26-34 days) is mostly due to variation in the follicular phase, not the luteal phase
• If conception occurs, hCG from the implanting blastocyst rescues the corpus luteum, sustaining progesterone production and preventing menstruation

The Basis of the Pregnancy Test: hCG

• hCG prevents premature degeneration (luteolysis) of the corpus luteum
• It sustains progesterone production from the corpus luteum through the first trimester
• This is why periods stop in pregnancy - the progesterone-driven endometrium is maintained

Timeline: When Does hCG Become Detectable?

Types of Pregnancy Tests

1. Qualitative Tests (Yes/No Result)

• Uses immunochromatography (lateral flow)
• Detection threshold typically 25 IU/L
• OTC home tests may detect as low as 0.5-25 IU/L (they use larger sample volumes)
• First-morning urine is preferred because it is concentrated
• Waived under CLIA '88 - can be done at home or point-of-care
• Result: colored test line = positive

• Slightly more sensitive than urine - detects pregnancy earlier because serum hCG concentration is higher than urine
• Classified as moderately complex under CLIA - requires centrifugation, must be done in a lab

2. Quantitative Tests (Serum beta-hCG Level)

• Measures exact hCG concentration (IU/L or mIU/mL)
• Detection threshold ~5 IU/L
• Used to:
  • Diagnose and date early pregnancy
  • Monitor pregnancy viability (serial doubling)
  • Diagnose ectopic pregnancy (combined with ultrasound)
  • Monitor trophoblastic disease (gestational trophoblastic tumors, choriocarcinoma)

Best Time to Perform a Pregnancy Test

False Positives and False Negatives

• Heterophile antibodies or human anti-mouse antibodies (HAMA) in serum
• Molar pregnancy or choriocarcinoma (very high hCG)
• Delayed reading of the test result (test band darkens over time)
• Rare: tuboovarian abscess (one case report of positive urine / normal serum beta-hCG)

• Testing too early (hCG below threshold)
• Dilute urine (not first-morning specimen)
• Hook effect - extremely high hCG (e.g., complete mole) can saturate antibodies and produce false negative on some assays
• hCG beta-core fragment in urine (predominant after 5th week) can competitively inhibit the test

Whole Blood as an Alternative Specimen

Summary Diagram

Pregnancy Test in Menstrual Cycle - Short Notes

What is Detected?

• All pregnancy tests detect beta-hCG (beta-human chorionic gonadotropin)
• Secreted by trophoblast cells after implantation of the blastocyst

hCG & The Menstrual Cycle

Role of hCG in Cycle

• Prevents luteolysis (corpus luteum degeneration)
• Maintains progesterone production → endometrium preserved → no menstruation

Types of Tests

Best Time to Test

• Day of missed period - ~50% sensitive
• 1 week after missed period - most reliable
• Use first-morning urine (concentrated = more hCG)

hCG Doubling Rule

• Doubles every ~2.5 days in first 4 weeks (viable IUP)
• Fails to rise 66% in 48h → likely nonviable / ectopic
• Peak: 100,000-200,000 mIU/mL at 10-14 weeks

False Results

Visual Pathways - Short Notes (Endocrine Focus)

The Visual Pathway - Anatomy

• Nasal retinal fibers (temporal visual field) → cross to contralateral optic tract
• Temporal retinal fibers (nasal visual field) → stay ipsilateral

Endocrine Relevance - The Optic Chiasm & Pituitary

Lesion Map & Visual Field Defects

The Key Endocrine-Visual Connection: Pituitary Adenoma

• Pituitary adenoma grows upward out of the sella turcica
• Compresses the crossing nasal fibers at the chiasm
• Results: Bitemporal hemianopia (both temporal fields lost)
• Often insidious - patient may not notice until late
• Always test each eye separately to detect it

Quick Memory Rule

Endocrine Causes of Chiasmal Compression

1. Pituitary adenoma (most common)
2. Craniopharyngioma
3. Hypothalamic glioma
4. Meningioma (tuberculum sellae / diaphragma sellae)
5. Rathke's cleft cyst
6. Aneurysm (internal carotid / anterior communicating artery)

Visual Pathway & Endocrine - Easy Words

What is the Visual Pathway?

The Road: Step by Step

Eyes (Retina)
     ↓
Optic Nerve (like a cable from each eye)
     ↓
Optic Chiasm (X-shaped crossing point)
     ↓
Optic Tract (road to brain)
     ↓
LGN - Lateral Geniculate Nucleus (relay station in thalamus)
     ↓
Optic Radiations (fan of fibres)
     ↓
Visual Cortex - Occipital lobe (back of brain = "seeing centre")

The Most Important Part - The Optic Chiasm

• The chiasm is shaped like the letter X
• Here, inner half fibres cross over to the opposite side
• Outer half fibres stay on the same side
• The chiasm sits just above the pituitary gland

This is why pituitary problems affect vision!

The Endocrine Connection (Most Important for Exams)

Pituitary Adenoma (most common cause)

• The pituitary gland sits in a bony cup called the sella turcica
• When a pituitary tumour grows upward, it squeezes the chiasm from below
• It hits the crossing fibres first (the inner fibres that carry outer field vision)
• Result: You lose both outer (temporal) visual fields
• This is called Bitemporal Hemianopia

Normal vision:         You see left + right + centre

Bitemporal hemianopia: You see only the centre
                       (like wearing blinkers / horse blinders)

Lesion Table in Simple Words

Key Endocrine Causes of Chiasm Compression

1. Pituitary adenoma - grows upward from below
2. Craniopharyngioma - grows near pituitary in children
3. Meningioma - tumour of brain covering near sella
4. Hypothalamic glioma - grows downward
5. Aneurysm - balloon in a nearby artery

The Golden Rule

One-Line Summary

The pituitary tumour grows up, squeezes the optic chiasm, destroys the crossing fibres, and causes bitemporal hemianopia - the patient loses both outer fields of vision.