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Sperm: It Only Takes One, Doesn't It?


a blog by Serena H. Chen, M.D., IRMS Reproductive Medicine at Saint Barnabas, April 22, 2011

Why do doctors say that a normal sperm concentration is 20 million sperm per milliliter? I just want one baby, maybe two. I don’t need millions of babies. Why do I need millions of sperm to conceive?

Well, the truth is, we don’t really know, but we have some ideas about why millions of sperm are required.

The normal ranges of sperm concentration, count, motility, etc., are derived from studying sperm in thousands of normal fertile males. Interestingly, the normal ranges have been changing over time. In the 1950s a normal sperm concentration was typically around 50 million sperm per milliliter. These days, most experts expect a concentration of only 20 million sperm per milliliter. There is evidence that in ancient times — such as in ancient Egypt — normal sperm concentrations may have been even higher.

After counting sperm in hundreds or thousands of semen samples from males of reproductive age who have fathered a child and have no history of infertility, scientists define the normal range as the middle 95 percent of values. The normal range can then be verified by seeing what kinds of sperm counts are seen in males who do have a history of infertility. The lower the count, the more difficult it appears to be to conceive.

When we study sperm in vitro (in the lab, outside the body), we can see that if we take the same amount of sperm cells from two different people — one with a normal semen analysis and one with an abnormal semen analysis — fertilization of eggs in the lab will be much lower with the sperm from the man with an abnormal semen analysis. In fact, before 1993 when ICSI (intracytoplasmic sperm injection) was invented, many men with low sperm counts had to use donor sperm in order to conceive. In vitro fertilization (IVF) just did not help with moderate to severely low sperm counts, even when you dropped hundreds of thousands of sperm right on top of the egg. ICSI — the process of injecting one sperm into one egg — has revolutionized the treatment of male infertility. Even men with almost no sperm at all can conceive using their own sperm through the use of ICSI.

So, it is not really about sperm number, it is about sperm function. The numbers are just a sign that something may be wrong. If the concentration is persistently low, this is a reflection of the health of the sperm and their ability to fertilize the egg. Remember that you need at least three semen analyses to decide whether or not you really have a problem. Everyone can have a bad day once in awhile. An abnormal semen analysis should be repeated before you draw any conclusions.

So the answer to “it only takes one sperm to make a baby, doesn’t it?” is “yes” and “no.”

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