Human chorionic gonadotropin (hCG) is a hormone produced by the embryo after conception. Human embryos can produce hCG as early as 8 days after fertilization. During IVF, however, final egg maturation is triggered by an hCG injection (generally 10,000 units) given approximately 36 hours prior to egg retrieval, increasing hCG levels in the patient's body. It takes approximately 14 days for the hCG from the trigger shot to be cleared (metabolized).
Prior to 14 days, it is not possible to differentiate between hCG from the trigger shot and hCG from the embryo. It is for this reason that most IVF programs will do a first pregnancy test approximately 15 days after the hCG trigger injection (or 13 days after egg retrieval).
They analyzed 918 transfers of fresh embryos that were performed over a three- year period. There were 431 clinical pregnancies (47%). Of these, 73% were singleton, 24% twin and 3% were triplet. Day 3 transfers produced higher hCG levels for both singleton and twin pregnancies compared with day 5 transfers (see Table above).
It is possible that the difference in hCG levels was due to early post-implantation losses. They therefore further analyzed hCG levels in twin pregnancies established by transferring only 2 embryos (see Table above). Once again, day 3 transfers resulted in higher hCG levels on both tests.
Why the Difference in hCG Levels?
So, why do pregnancies resulting from day 3 transfers have hCG levels that are about 50% higher in contrast to those resulting from day 5 transfers? Early subclinical pregnancy losses could be a possible explanation. Gender bias was also discussed as a possible mechanism – there were more males in the blastocyst group. It is also possible that the additional 2 days of culture in the laboratory might directly and adversely affect the normal development of the embryos, resulting in subtle delay or reduction in hCG production.
The hCG levels above are only guidelines. There are always exceptions to the rule. This is why, even if the hCG levels are low it is important to continue the medications and keep follow up monitoring appointments.
Dr. Karande is Board Certified in the specialty of Obstetrics and Gynecology as well as the subspecialty of Reproductive Endocrinology and Infertility. He is a Fellow of the American College of Obstetricians and Gynecologists and Member of the American Society for Reproductive Medicine.