Why Human chorionic gonadotropin (hCG) for Pregnancy tests

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

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 

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.

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).

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).

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.