Why is Human chorionic gonadotropin (hCG) done for Pregnancy tests?

Human chorionic gonadotropin - The basis of pregnancy tests and Clinical implications

a pregnant woman

Table of Contents (toc)

When is hCG first detectable?

 Detection of human chorionic gonadotropin (hCG) in blood or urine is the basis of all pregnancy tests

hCG is secreted into the maternal circulation after implantation, which may occur as early as 6 days after ovulation but typically occurs 8 to 10 days after ovulation 

When Does Ovulation Occur

Ovulation occurs around day 14 of a 28-day menstrual cycle.

This is the earliest that hCG can be detected with a standard serum hCG test. However, the ovulation-to-implantation interval has been observed to vary by up to six days in naturally conceived pregnancies.

What delayes pregnancy test to come positive?

Late implantation delays the time to a positive pregnancy test and has been associated with an increased risk of pregnancy loss.

In a study of females with normal menstrual cycles who were attempting to conceive, the median hCG concentration on the first day of expected but missed menses (ie, approximately 4 weeks of gestation) was 239 milli-international units/mL in serum and 49 milli-international units/mL in a spot urine, but there was a wide range among individuals.

The range of hCG values was narrower in a study of over 4400 females who conceived by in vitro fertilization, underwent embryo transfer two to three days after egg retrieval, and had at least one viable embryo at 8 weeks of gestation: the median hCG concentration on day 12 after embryo transfer/day 16 after ovulation (ie, approximately 4 weeks of gestation) was 118 milli-international units/mL (interquartile range 98 milli-international units/mL).

Some common symptoms of pregnancy:

• Missed Period: One of the earliest signs, especially if you have regular menstrual cycles.
• Nausea and Vomiting: Often referred to as "morning sickness," this can occur at any time of day.
• Breast Changes: Tenderness, swelling, or darkening of the areolas.
• Fatigue: Increased tiredness and a need for more sleep.
• Frequent Urination: An increase in urination due to hormonal changes and pressure on the bladder.
• Food Cravings or Aversions: Changes in taste preferences or aversion to certain foods.
• Mood Swings: Emotional fluctuations due to hormonal changes.
• Bloating: A feeling of fullness or swelling in the abdomen.

Signs of pregnancy list out for medical students

Presumptive Signs

• Missed Period
• Nausea and Vomiting
• Breast Changes
• Fatigue
• Frequent Urination
• Food Cravings or Aversions
• Mood Swings
• Bloating

Probable Signs

• Positive Home Pregnancy Test
• Uterine Enlargement
• Chadwick's Sign (bluish discoloration of cervix)
• Hegar's Sign (softening of the uterus)
• Fetal Heartbeat Detected by Doppler

Positive Signs

• Visualization of Fetus on Ultrasound
• Detection of Fetal Heartbeat via Ultrasound
• Blood Tests Confirming hCG Levels

How does hCG level change across pregnancy 

The hCG concentration doubles every 29 to 53 hours during the first 30 days after implantation of a viable, intrauterine pregnancy; a slower rise is suggestive of an abnormal pregnancy (eg, ectopic, early embryonic death).

positive pregnancytest results

The hCG concentration

The hCG concentration peaks at 8 to 10 weeks of gestation, averaging 60,000 to 90,000 milli-international units/mL at that time, but again, the range of normal is quite wide (5000 to 150,000 milli-international units/mL or more).

Thus, hCG levels are not useful for estimating gestational age, except in the first one to three weeks postconception

After 8 to 10 weeks of gestation, hCG levels decline, reaching a median concentration of approximately 12,000 milli-international units/mL at 20 weeks, again with a wide range of normal: 2000 to 50,000 milli-international units/mL or more.

hCG concentration stays relatively constant from approximately the 20th week until term.

Variation in hCG Level

The factors accounting for the wide variation in interindividual hCG levels have not been studied extensively in accurately dated pregnancies; maternal weight accounts for some of the variation throughout pregnancy.

In some cases, an unrecognized vanishing twin affects the hCG level.