question: Is the fastest sperm to reach and fertilise the egg necessarily the fittest or healthiest? If not, what usually determines its success?
comment: Mostly luck. It's not necessary fittest or healthiest of all, because most sperm cells aren't lucky by definition of luck. A group of sperm cells will move together and many of them should die out just to make path to their target easier for other cells. But to fertilize the egg, they should be healthy enough and to complete some "checks". That is like a basic check that their DNA is not damaged badly, under the point when cells become unable to do it's job, but not enough to ensure that DNA don't have any mutation - they almost certainly will have some minor mutations.
passages: The main potential factors that influence pregnancy (and live birth) rates in IVF have been suggested to be maternal age, duration of infertility or subfertility, bFSH and number of oocytes, all reflecting ovarian function. Optimal woman's age is 23–39 years at time of treatment.
  
  Biomarkers that affect the pregnancy chances of IVF include:
  
  BULLET::::- Antral follicle count, with higher count giving higher success rates.
  
  BULLET::::- Anti-Müllerian hormone levels, with higher levels indicating higher chances of pregnancy, as well as of live birth after IVF, even after adjusting for age.
  
  BULLET::::- Factors of semen quality for the sperm provider.
   IVF success rates are the percentage of all IVF procedures that result in a favourable outcome. Depending on the type of calculation used, this outcome may represent the number of confirmed pregnancies, called the pregnancy rate, or the number of live births, called the live birth rate. The success rate depends on variable factors such as maternal age, cause of infertility, embryo status, reproductive history, and lifestyle factors.
  
  Maternal age: Younger candidates of IVF are more likely to get pregnant. Women older than 41 are more likely to get pregnant with a donor egg.
   Section::::Related end-points.
  
  In some cases, success rates include delivery or presence of a live baby (preferably specified as "delivery rate" or "live birth rate" respectively).
  
  Section::::Related end-points.:Fertilization rate.
  
  In IVF or its derivatives, "fertilization rate" may be used to measure how many oocytes become fertilized by sperm cells. A fertilization rate of zero in one cycle, where no oocytes become fertilized, is termed a "total fertilization failure". Repeated ICSI treatment may be useful or necessary in couples with total fertilization failure.
  
  Section::::Related end-points.:Implantation rate.
   In men with a normal level of sperm production (normozoospermia), the percentage of sperm DNA fragmentation is positively correlated with age, and inversely correlated with progressive sperm motility. 
  
  No age related effects on sperm were noted in separate control groups recruited in different geographical locations, indicating that dietary habits, lifestyle or ethnicity could play a part in the quality of sperm.
   Section::::Sperm in animals.:Sperm size.
  
  Related to sperm quality is sperm size, at least in some animals. For instance, the sperm of some species of fruit fly ("Drosophila") are up to 5.8 cm long — about 20 times as long as the fly itself. Longer sperm cells are better than their shorter counterparts at displacing competitors from the female’s seminal receptacle. The benefit to females is that only healthy males carry ‘good’ genes that can produce long sperm in sufficient quantities to outcompete their competitors.
  
  Section::::Sperm in animals.:Market for human sperm.
  
  Some sperm banks hold up to of sperm.
   The pregnancy rate also depends on the total sperm count, or, more specifically, the total motile sperm count (TMSC), used in a cycle. The success rate increases with increasing TMSC, but only up to a certain count, when other factors become limiting to success. The summed pregnancy rate of two cycles using a TMSC of 5 million (may be a TSC of ~10 million on graph) in each cycle is substantially higher than one single cycle using a TMSC of 10 million. However, although more cost-efficient, using a lower TMSC also increases the average time taken to achieve pregnancy. Women whose age is becoming a major factor in fertility may not want to spend that extra time.
   The live birth rate is the percentage of all IVF cycles that lead to a live birth. This rate does not include miscarriage or stillbirth; multiple-order births, such as twins and triplets, are counted as one pregnancy. A 2012 summary compiled by the Society for Reproductive Medicine which reports the average IVF success rates in the United States per age group using non-donor eggs compiled the following data:
   Without technological intervention, a non-motile or abnormally-motile sperm is not going to fertilize. Therefore, the fraction of a sperm population that is motile is widely used as a measure of semen quality . Insufficient sperm motility is a common cause of subfertility or infertility. Several measures are available to improve sperm quality.
  
  Section::::Sperm DNA damage.
  
  Sperm DNA damage is common in infertile men. About 31% of men with sperm motility defects have high levels of sperm DNA fragmentation.
  
  Section::::Classifications of motility.
  
  BULLET::::1. Straight moving,
  
  BULLET::::2. Zig-zag moving,
  
  BULLET::::3. Vibrating,
  
  BULLET::::4. Non-motile
  
  Section::::External links.
  
  BULLET::::- Sperm motility, Colorado State University
   Calving ease predicts the level of difficulty first time heifers will have during birth. These are determined by the percentage of unassisted births for that particular animal.
   Sperm competition is the competition that occurs when females store sperm, of varying males, in their spermatheca and use this sperm to fertilize their eggs. This type of competition is prevalent when females mate more than once in which they store sperm in a viable condition and have sperm from previous matings present when they re-mate
   There are many factors that influence the sperm quality. Exposure to any of the temporary factors can cause up to a three-month delay before sperm quality returns to normal, due to spermiogenesis.
  
  Section::::Factors.:General decline.
   IVF attempts in multiple cycles result in increased cumulative live birth rates. Depending on the demographic group, one study reported 45% to 53% for three attempts, and 51% to 71% to 80% for six attempts.
  
  Section::::Medical uses.:Success rates.:Pregnancy rate.
  
  Pregnancy rate may be defined in various ways. In the United States, the pregnancy rate used by the Society for Assisted Reproductive Technology and the Centers for Disease Control (and appearing in the table in the Success Rates section above) are based on fetal heart motion observed in ultrasound examinations.
   Having all eggs penetrated by multiple sperm is considered to be a positive sign for fertility. Results suggest that men whose sperm fail the hamster test are a third as fertile than those whose sperm pass.
  
  Donated sperm and intracytoplasmic sperm injection techniques are considered more strongly for those whose sperm fail this test.
   There is no universally accepted definition of the term. Thus in IVF pregnancy rates may be based on initiated treatment cycles, cycles that underwent oocyte retrieval, or cycles where an embryo transfer was performed. In terms of outcome, "pregnancy" may refer to a positive pregnancy test, evidence of a pregnancy with a "viable" fetus or implantation. Furthermore, pregnancy rates can be influenced in IVF by transferring multiple embryos that may result in multiple births. A strict definition in the IVF setting would refer to the singleton pregnancy rate that determines how many live singletons are born in relation to initiated IVF cycles.
   The body has compensatory mechanisms, like the cremaster muscle relaxing and letting the testicle hang further away from the warm body, sweating and a countercurrent exchange of blood cooling inflowing blood. However, despite these compensations, there are activities that should not be performed too often, in order to prevent infertility due to heat;
  
  BULLET::::- sauna sessions
  
  BULLET::::- bathing for a long time in hot water
  
  BULLET::::- Long-time tanning bed sessions
  
  BULLET::::- Placement of a laptop computer over the groin for extended use
  
  Fever raises the body temperature, which can strike sperm quality.
   Section::::Tests.:Hamster zona-free ovum test.
  
  A man's sperm are mixed with hamster eggs that have had the zona pellucida (outer membranes) removed, and the number of sperm penetrations per egg is measured. No strong correlation has been found between hamster egg penetration rates and the various semen parameters and the role of the hamster egg penetration test in the investigation of the causes of infertility should be evaluated further. However, a negative result on the hamster test correlates with a lower probability of the man's partner becoming pregnant.
  
  Section::::Tests.:Antisperm antibodies test.
   Section::::Mate choice.
  
  The "good sperm hypothesis" is very common in polyandrous mating systems. The "good sperm hypothesis" suggests that a male's genetic makeup will determine the level of his competitiveness in sperm competition. When a male has "good sperm" he is able to father more viable offspring than males that do not have the "good sperm" genes. Females may select males that have these superior "good sperm" genes because it means that their offspring will be more viable and will inherit the "good sperm" genes which will increase their fitness levels when their sperm competes.
   The World Health Organization has a value of 50% and this must be measured within 60 minutes of collection. WHO also has a parameter of "vitality", with a lower reference limit of 60% live spermatozoa. A man can have a total number of sperm far over the limit of 20 million sperm cells per milliliter, but still have bad quality because too few of them are motile. However, if the sperm count is very high, then a low motility (for example, less than 60%) might not matter, because the fraction might still be more than 8 million per millilitre. The other way around, a man can have a sperm count far less than 20 million sperm cells per millilitre and still have good motility, if more than 60% of those observed sperm cells show good forward movement - which is beneficial because nature favours quality over quantity.
   MHC-mediated mate choice may occur after copulation, at the gametic level, through sperm competition or female cryptic choice. The Atlantic salmon, "Salmo salar", is one species in which sperm competition is influenced by the variation in the major histocompatibility complex, specifically that of the Class I alleles. Atlantic salmon males have higher rates of successful fertilization when competing for eggs from females genetically similar at the class I genes of the MHC.
  
  Another species that exhibits MHC-associated cryptic choice is the Arctic charr "Salvelinus alpinus". In this case, however, it seems that sperm selection is more dependent on the ovum.
   Even within ejaculates, sperm have adopted different morphologies. It has not been answered by the current scholarship if these different morphologies offer advantages in fertilization, but we can still model them as trade-offs (ATP expenditure vs. Longer tail) and as altruism (pairing) between sperm brothers. The next question ask is whether these differential strategies show any trends. If certain morphologies tend to become more common, it might mean that there is competition within sperm in the sense that not every sperm has the same chance of reaching the egg.
   BULLET::::- Grade d: These are immotile and fail to move at all. Sometimes also denoted motility I.
  
  Section::::Parameters.:Morphology.
  
  Regarding sperm morphology, the WHO criteria as described in 2010 state that a sample is normal (samples from men whose partners had a pregnancy in the last 12 months) if 4% (or 5th centile) or more of the observed sperm have normal morphology.
  
  Morphology is a predictor of success in fertilizing oocytes during in vitro fertilization.
  
  Up to 10% of all spermatozoa have observable defects and as such are disadvantaged in terms of fertilising an oocyte.
   BULLET::::- "r" is the pregnancy rate per cycle, between 10% to 35%
  
  BULLET::::- "n" is the total motile sperm count recommended for vaginal insemination (VI) or intra-cervical insemination (ICI), approximately 20 million pr. ml.
  
  The pregnancy rate increases with increasing number of motile sperm used, but only up to a certain degree, when other factors become limiting instead.
  
  With these numbers, one sample would on average help giving rise to 0.1–0.6 children, that is, it actually takes on average 2–5 samples to make a child.
  
  For intrauterine insemination, a "centrifugation fraction" ("f") may be added to the equation:
   Males carrying Robertsonian translocations in their chromosomes have significantly higher degree of sperm cell apoptosis and lower concentration. Sperm cells also have decreased forward motility, but normal morphology.
  
  Testicular cancer, Leydig cell tumours and Sertoli cell tumours are also associated with reduced sperm count and quality.
  
  Section::::Tests.
  
  Section::::Tests.:Semen analysis.
   Because not each IVF cycle that is started will lead to oocyte retrieval or embryo transfer, reports of live birth rates need to specify the denominator, namely IVF cycles started, IVF retrievals, or embryo transfers. The Society for Assisted Reproductive Technology (SART) summarised 2008-9 success rates for US clinics for fresh embryo cycles that did not involve donor eggs and gave live birth rates by the age of the prospective mother, with a peak at 41.3% per cycle started and 47.3% per embryo transfer for patients under 35 years of age.
   BULLET::::- Many psychoactive drugs, including many antidepressants, many antiepileptics (e.g. lithium), and propranolol
  
  BULLET::::- Opioid analgesics
  
  BULLET::::- Calcium channel blockers
  
  BULLET::::- Phosphodiesterase inhibitors (e.g. caffeine, theophylline, pentoxifylline)
  
  BULLET::::- Statins
  
  BULLET::::- Calcium chelators (e.g. EDTA)
  
  Also, numerous products that are intended for exposure to spermatozoa have only a general assumption of safety based on the absence of evidence of actual harm.
  
  Section::::Factors.:Chemicals.:Hormones.
  
  BULLET::::- Anabolic steroids use and use of other hormones can reduce sperm quality. Changes in hormone homeostasis affect the spermatogenesis.
  
  The body also has natural variations in hormone concentrations, giving sperm quality natural fluctuations as well.
  
  Section::::Factors.:Chemicals.:Diet.
  