The Real Meaning of Beta hCG Levels

13 Jul
Beta hCG Pregnancy Test

Certainly the most anticipated lab test in a fertility clinic is a human chorionic gonadotropin (hCG) level. While everyone knows that having a positive beta hCG is the ultimate prize, it is far more difficult to truly understand what the test is really telling you.

First, what does a positive (+) beta hCG test really mean? Here we must differentiate between a urine test and a blood pregnancy test. The only real difference is the sensitivity of the test, meaning how high does the hCG level actually have to be to get detected. On a home pregnancy test, the lowest level routinely detected varies between 20-30 mIU/ml. As these levels are achieved fairly early in pregnancy – in fact by the time the expected period is missed – the levels should be above this detection level.

A blood test, on the other hand, is a more vigorous tool and can detect hCG very accurately as low as 5 mIU/ml. Levels below 5 can be detected, but the accuracy of these evaluations suffers greatly. These levels can be detected even before the missed period, hence their usage in IVF cycles. One thing that must be understood here is that these tests cannot distinguish between hCG created by the pregnancy or that given by injection. One must therefore be careful interpreting levels that have been drawn close to the time of the hCG trigger of ovulation/maturity. Twelve days is generally enough time to allow what was injected to be completely metabolized and cleared.

Second, how high must the beta hCG level be before we can actually expect to see something on ultrasound? Once upon a time, when the world was young and I was fresh out of training, we did primarily abdominal scanning. The hCG level most commensurate with seeing the pregnancy was very high, about 6500 mIU/ml. Since the advent of the trans vaginal probe and the great increase in the ability to see things in the uterus, this “U/S detectable” level decreases to about 1000 mIU/ml. This can be equated to about 6 weeks gestation (remember, we always add two weeks to go all the way back to the start of the cycle, not from the time of implantation).

Beta hCG Levels

The hardest question to answer is how fast these levels should increase. A common rule of thumb is that on average they should approximately double every two days. But remember that these are averages. When following such numbers, we must use curves like the one shown above. The only truly abnormal parts of the curve are the upper 2.5% and the lower 2.5%. All the other values strung between these two points are still normal, they are simply more or less than their neighbor. It is the same with hCG levels. If the number is not doubling, that is not a reason to “hang crepe” and panic. It is still most probably normal.

Additionally, the doubling time (time needed for the value to double) also changes as the pregnancy advances, hence you can expect the increase to slow down as you follow it. In fact, once the thresholds for ultrasound detection have been passed, following serial hCG levels does little except overly worry people. Again if I had a nickel for every baby I have held in my arms that did not initially have perfectly doubling hCG levels or for every D & C I have had to perform for a failed pregnancy that was indeed doubling, I would be retired in the South of France reading poetry and eating peeled grapes. At a bottom line, do not hesitate to ask your coordinator or physician about their ideas concerning these levels so you do not unduly worry yourselves.

1,229 Comments

  • cheyenne says:

    Last period was March 2014, became pregnant the following month and delivered a healthy 36 week baby Jan 2015. At my 6 week check i got the nexplanon but had it removed at the end of july, in hopes of trying for another. He is almost 8 months and I have pumped and bottle fed him breastmilk since birth. I have not had a period but knew I was pregnant.
    Aug 9 I had a little bloody DC, and again Aug 20, 21, 22. Aug 19 I took a HPT and seen a very faint line. I began taking more hpt nearly every day since and seen the line get a little more visable each day. This morning my line was so faint I could barely see it so called my ob and had a blood test. Beta hcg showed my level was 3.5 I was told I am either extremely early or it is a chemical pregnancy. I go back next friday to recheck .
    This is my 3rd pregnancy, my last pregnancy I tested and seen my ob for blood work to confirm and ended up being less than 2 weeks. I am being optimistic and trying to tell myself that my body is just more sensitive to pregnancy since it is my 3rd and I knew sooner with this one, but I am afraid.
    Also. After 2 hours I looked at my test I took this morning and the very faint line was a dark purple.
    I don’t understand how I am getting positive hpt with a 3.5

    • Geoffrey Sher says:

      A 3.5u hCG level will not produce a dark line. Go in on Monday and have a quantitative blood hCG test done.

      Good luck!

      Geoff sher

  • christy says:

    Two days ago I got an extremely barely faint positive at 10 dpo. The next two days all my pg tests were negative. I called my ob and they did a blood hcg test and it came back at a 3. I am assuming I had a chemical pregnancy and that my period will start in 2 days? The nurse thought it odd that my number was a 3. I have been on progesterone suppositories since cd 16 – I am assuming I should stop taking them with a number so low? Just wondering what your thoughts were on this! Thanks

    • Geoffrey Sher says:

      Hi Christy,

      I am sorry about the outcome. It likely means that you will need (after careful reappraisal of factors that can affect egg quality and implantation) to try again.

      Good luck!

      Geoff Sher

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