Beta-hCG

Beta-hCG (bHCG or β-hCG) is a sex hormone found in the mother's blood serum that can be used to help interpret obstetric ultrasound findings.

Beta-hCG levels may be used in three ways in the clinical setting of pregnancy:

1. qualitatively, for presence/absence of fetal tissue
   • more often determined with a urine test than with a serum test
   • includes or excludes a pregnancy-related differential in a female pelvic ultrasound
2. quantitatively, for assessing if a pregnancy is meeting appropriate cutoff values for its age
   • has been used in the first trimester to help determine if an ectopic pregnancy should be suspected
3. quantitatively, for trending the growth rate of fetal tissue
   • has been used in the first trimester to help determine if a pregnancy is failing
   • has been used to differentiate retained products of conception (RPOC) from gestational trophoblastic disease

ItOutside the pregnancy setting, it may also be used quantitatively, as a tumour marker, to aid in diagnosing a potentially beta-hCG secreting mass (e.g. choriocarcinoma).

Terminology

"hCG" is an initialism for "human chorionic gonadotropin"

• chorionic: produced. Chorionic refers to the production of hCG from the syncytiotrophoblasts of the chorion, which which are developing into the fetal placenta
• gonadotropin:. Gonadotropin refers to a substance that regulates gonadal activity (FSH(for instance FSH and LHL, anterior the two anterior pituitary gonadotropins, are in this class)

hCG consists of two moieties:

• an alpha (α) moiety that is similar to FSH, LH, and TSH,

and and a

• a beta (β) moiety that is unique to human chorionic gonadotropin

In view of this homology between hCG and other gonadotropins, all commercially-available assay kits test for the beta subunit only. This is to reduce the risk of inadvertently mistaking FH and LSH as part of the hCG fraction. 

Markedly elevated beta-hCG levels may occur with gestational trophoblastic disease, whilst an abundance of the alpha subunit (of hCG) may cause hyperthyroidism ⁷.

The physiologic role of the hormone is to maintain the corpus luteum, thereby maintaining a favourable intrauterine environment for pregnancy.

NB: gonadotropin is the correct spelling; gonadotrophin is an oldantiquated spelling that should be avoided.

Differential diagnosis

Quantitative for gestational sac visualisation

The use of beta-hCG values to guide interpretation of first-trimester ultrasound findings has a long history. "Discriminatory levels" were first devised to indicate at what beta-hCG level a gestational sac should be seen ². These levels are continually revised (values are in mIU/mL):

• >0
  • fetal chorionic/placental tissue is present somewhere
• >1000-2000
  • older cut off for visualisation of an intrauterine gestational sac
• >2000
  • non-visualisation of the intrauterine gestational sac is suspicious for an ectopic pregnancy but does not exclude a normal intrauterine pregnancy (IUP) ³

Currently, it is discouraged to treat based on a single beta-hCG level in a hemodynamically-stable woman with a pregnancy of uncertain location ³.

Quantitative for trending

Beta-hCG values can also be used to determine if a pregnancy is progressing appropriately (until about ~10 weeks gestational age)

• <8 weeks of pregnancy (gestational age)
  • beta-hCG level is assumed to double every ~48 hours ⁴
• 8-10 weeks of pregnancy
  • beta-hCG level is assumed to double approximately every ~5 days ⁴

A pregnancy with vaginal bleeding and a beta-hCG level that is not increasing appropriately (or declining) is assumed to be an inevitable miscarriage ⁵, although solidity of this idea after 8 weeks and above a level of 5000 mIU/ml has been questioned ⁶.

Trending beta-hCG levels can help differentiate a cervical ectopic pregnancy from an inevitable miscarriage.

Beta-hCG levels decrease rapidly in the first 1-2 weeks after delivery, termination or complete miscarriage and then the rate of decrease slows down beyond 2 weeks. At 6 weeks levels should be zero.

Other

Gestational trophoblastic disease (e.g. complete mole) is associated with very elevated beta-hCG levels.

On ultrasound, the gestational trophoblastic disease may appear radiologically similar to retained products of conception (RPOC), but the two may be differentiated with beta-hCG.

• gestational trophoblastic disease: markedly elevated beta-hCG levels
• retained products of conception: falling beta-hCG levels

Beta-hCG levels may also be raised by some ovarian neoplasms (e.g. ovarian embryonal carcinoma, ovarian dysgerminoma), as well as in testicular germ cell tumours ⁸. 

See also

• tumour markers