Every ejaculate contains a mix of well-formed and oddly shaped sperm. Morphology is the lab's estimate of how many look "normal": an oval head of the right size with a proper cap (the acrosome), a neat midpiece and a single, uncoiled tail. Sperm with a misshapen head, a kinked midpiece or a double tail count as abnormal.
Modern labs use strict criteria (often called Kruger or Tygerberg criteria), which set such a high bar that only a small minority of sperm pass. That is why the numbers look alarming to anyone seeing them for the first time. When normal forms fall below the reference limit, the report may say teratozoospermia.
The WHO 2021 lower reference limit is 4% normal forms, the 5th centile among more than 3,500 men whose partners had recently conceived.1 In other words, a fertile man can have 96% of his sperm classed as abnormal and still sit within the reference range.
What is a normal sperm morphology result?
Four per cent normal forms or more is at or above the WHO lower reference limit. Seeing that most of your sperm are "abnormal" is expected and not, on its own, a problem.
Strict criteria were designed to pick out the best-shaped sperm, not to describe what a typical sample looks like. A lower reference limit of 4% means that plenty of recently fertile men have only a small percentage of normally shaped sperm, and the authors of the 2021 values stress that the limits are not a hard boundary between fertile and subfertile men.1
Where did the 4% figure come from?
It traces back to IVF research in the 1980s using strict criteria. Men with fewer than 4% normal forms had much lower fertilisation rates in the lab.
In a 1988 study from Tygerberg Hospital in South Africa, men with acceptable count and motility but fewer than 4% normal forms (and a low morphology index) had just 7.6% of eggs fertilised in IVF.2 Men with 4% to 14% normal forms had a fertilisation rate of 63.9%, and those above 14% fertilised within the lab's normal range.2
That work made strict morphology a standard part of the semen analysis. The WHO's population-based figures later landed on the same 4% threshold as a lower reference limit.1
How much does morphology matter for natural conception?
It matters, but it is not decisive on its own. Morphology is one of the more useful predictors in studies, yet fertile and infertile men overlap a great deal.
In a US study of 1,461 men, the percentage of normally shaped sperm was the best of the three standard measures at telling fertile and infertile men apart.3 It classed below 9% normal forms as subfertile and above 12% as fertile, but even morphology was not a powerful discriminator, and none of the measures was diagnostic.3
A Danish study of 430 couples trying for their first pregnancy found the proportion of normally shaped sperm was strongly linked to the chance of conceiving, independently of sperm concentration.4
Should I worry about a low morphology score?
Not on the basis of one result. Repeat the test, look at the other parameters, and speak to a doctor if the pattern persists or you are struggling to conceive.
Morphology is judged by eye, so it is worth asking which criteria your lab used and comparing results from the same lab where possible. A low score alongside normal count and motility carries different weight from a low score across the board.
Talk to your GP about referral to a urologist or fertility clinic if you and your partner have been trying without success, if several parameters are low, or if you have symptoms such as testicular pain or swelling. Some clinics also offer sperm DNA fragmentation testing, which measures something different from shape.
Common questions
Does abnormal sperm shape cause birth defects?
Morphology describes the size and shape of sperm under a microscope, and none of the studies cited here tested it as a measure of a baby's health. If you are worried, ask a fertility specialist to explain your result in context.
Is 2% morphology bad?
It is below the WHO 2021 lower reference limit of 4%, so it would be flagged as low.1 Many men with low morphology still conceive, and the result should be repeated and read alongside count and motility.
Can sperm morphology improve?
Results do vary from one sample to the next, so a single low score may not be your usual level. Because sperm take weeks to form, give any lifestyle change a couple of months before retesting.
Read more on this
Lifestyle factors that affect male fertility: a practical guide
How long does it take sperm to regenerate?
Sources
- Campbell MJ, Lotti F, Baldi E, et al. Distribution of semen examination results 2020 - A follow up of data collated for the WHO semen analysis manual 2010. Andrology. 2021;9(3):817-822. PubMed
- Kruger TF, Acosta AA, Simmons KF, et al. Predictive value of abnormal sperm morphology in in vitro fertilization. Fertil Steril. 1988;49(1):112-117. PubMed
- Guzick DS, Overstreet JW, Factor-Litvak P, et al. Sperm morphology, motility, and concentration in fertile and infertile men. N Engl J Med. 2001;345(19):1388-1393. PubMed
- Bonde JP, Ernst E, Jensen TK, et al. Relation between semen quality and fertility: a population-based study of 430 first-pregnancy planners. Lancet. 1998;352(9135):1172-1177. PubMed
Not medical advice. Written by Eric Stephens from the research cited above; not reviewed by a doctor. If something worries you, see a GP.