Human chorionic Gonadotropin, or hCG, plays an important role in maintaining early pregnancies. Blood and urine pregnancy tests evaluate levels of hCG produced in the early stages of pregnancy by placental tissue. If you're having twins, triplets or more, your initial hCG levels and subsequent levels may differ from those of a singleton pregnancy.
Production
Multiple pregnancies don't produce hCG any earlier than singleton pregnancies, since hCG isn't produced until the embryos implant, about six to 12 days after ovulation. According to a study reported in the "New England Journal of Medicine" by lead author Allen Wilcox, M.D., most embryos implant on day eight, nine or 10 after ovulation, or between four to six days before the day a new menstrual period would be expected. Levels of hCG at that time are too low for detection with over-the counter pregnancy tests. For the first few days after implantation, hCG levels may be about the same whether you're having one baby or several, What To Expect states.
Levels
An hCG level of 5milli-International Units per millilters or more indicates a positive pregnancy test, although in most pregnancies, the hCG level at the time of the first missed period measures 25 mIU/ml or more, according to the American Pregnancy Association. If you're having twins, triplets or more, your initial hCG level may be higher. A study reported in the "Journal of Assisted Reproduction and Genetics" by B. Guth in 1995 found that hCG levels in women having multiples were more likely to have hCG levels of 600 mIU/ml or higher 14 days after embryo transfer, as opposed to women with singleton pregnancies who had hCG levels of 200 mIU/ml or higher.
Rise
The growing placenta produces hCG, which normally doubles every two to three days in the first weeks of a singleton pregnancy. In a multiple pregnancy, levels may rise more quickly. Blood hCG levels are often 30 to 50 percent higher in women having multiples, but may not be any higher until after the first period is missed, What To Expect states, because it takes a few days for the difference to appear. So hCG levels may start out normal, then rise quickly.
Concerns
Other factors besides multiples can cause a rapid rise in hCG levels. Pregnancies that implant in the ovary may cause very rapidly rising hCG levels. Abnormal pregnancies called hydatidiform moles contain no embryo, but have faster than normal placental growth that creates very high hCG levels. Cancer that arises from placental tissue, called choriocarcinoma, can also cause rapidly rises hCG levels.
Diagnosis
A high hCG level or one that rises quickly doesn't always indicate that you're having multiples. Diagnosing a multiple pregnancy can only be done once the early embryonic structures are large enough to be seen on ultrasound, usually between the fifth and sixth weeks of pregnancy.
References
- American Pregnancy Association: human Chorionic Gonadotropin: The Pregnancy Hormone
- "Journal of Assisted Reproduction and Genetics": Predictive Value of HCG Levels 14 Days After Embryo Transfer; B Guth et al; Advanced Fertility Institute, Indianapolis, Indiana; 1995
- "New England Journal of Medicine": Time of Implantation of the Conceptus and Loss of Pregnancy; Allen Wilcox, M.D et al.; 1999
- University of Michigan Health System: Human Chorionic Gonadotropin
- What To Expect: Early HCG Levels and Twin Possibilities
