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In order for a woman to become pregnant, sperm is needed. Why is sperm count so important for pregnancy?

http://en.wikipedia.org/wiki/Sperm

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  • $\begingroup$ There are so many things that go into this issue: Sperm count Ability of sperm to swim (motility) Velocity or forward progression of the sperm Size and shape of the sperm (morphology) Total semen volume The liquefaction of the semen (the ability to go from normal gel-like state at ejaculation to a liquid state) Read more here $\endgroup$ – user16365 Jun 23 '15 at 2:30
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It's more than just sperm count that matters for a pregnancy if you are considering normal methods of insemination such as intercourse or assisted methods such as artificial/intrauterine insemination or in vitro fertilization. For couples having trouble getting pregnant, most OB-GYNs and/or Reproductive Endocrinologists/Infertility physicians do a semen analysis. This involves checking several things:

  1. Ejaculate Volume (>1.5mL)
  2. Sperm Concentration (>15million/mL)
  3. Motility (>40%)
  4. Morphology (>4%)

(and a host of other factors including rapid progressive motility, pH, etc). Why? Because there is no point of there being high numbers of sperm, if their motility is poor, because they won't be able to reach the oocyte. There's no point if they can reach an oocyte, if they are poorly shaped and cannot penetrate it. There's also no point in there being a high concentration of sperm, if the ejaculate volume is so low that it thins out before reaching the cervix. All these factors matter.

Total Motile Sperm Count (TMSC) is the most common way that physicians determine what route is best to increase chances for getting their patients pregnant.

TMSC = Concentration x Ejaculate Volume x % Motile (in decimals). Thus if, the value is:

  • greater than 10 million = normal male factor; check anatomy, check female causes
  • less than 10 million = low male factor; consider intrauterine insemination (IUI)
  • less than 5 million = severe male factor; consider in vitro fertilization (IVF) with intracytoplasmic sperm injection (ICSI)

Sources:

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As sort of a generic textbook answer, there's a number of factors. The vagina tends to be acidic, much so it's within a range of intolerance for sperm. To counteract this, seminal fluid is often alkaline in nature (1). The cervix also tends to be covered in a mucus that sperm must penetrate to enter the uterus. On top of factors like sperm lifespan, tolerance to vaginal environment, etc. the necessary numbers to actually achieve fertilization is important.

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