Library books on orange shelving
By Lee Hullender Rubin, DAoM, MS, LAc, FABORM
If you're trying to conceive, you've probably heard a lot about egg quality. You may have heard much less about sperm. Maybe you're wondering how much each one really matters, or why an embryo that looked good didn't lead to a pregnancy.
The short answer
Both egg and sperm matter. On average, the egg has the larger influence on embryo quality and implantation, largely because most chromosome errors in embryos come from the egg. Sperm is still a significant partner in conception. It supplies half the genetic instructions, helps start development, and carries genes that help build the placenta. Sperm problems can affect embryo growth, implantation, and pregnancy loss, especially when sperm DNA is damaged.
Think of an embryo as a set of books
Here's a simple way to picture it. Imagine every embryo starts with a set of instruction books for “building” a person.
The egg brings half the books, plus the library staff, the supplies, and the energy to get started. The sperm brings the other half of the books, plus the key that opens the library.
Your body needs every book in the set, with every page intact, arranged the right way. Most problems in early pregnancy come down to one of three things:
A missing or extra book. This is a whole chromosome error. Doctors call it aneuploidy.
A missing or extra chapter. This is a partial chromosome error, sometimes called a segmental error.
Torn pages. This is damaged DNA, which in sperm is called DNA fragmentation.
What happens in the first week
Day 0. The sperm enters the egg and turns development on.
Days 1 to 3. The embryo divides from one cell to about eight. It mostly runs on supplies the egg packed ahead of time. This is why sperm problems are often hidden at first, and an embryo can look healthy on day 3 even when sperm DNA is damaged.
Around day 3. The embryo starts reading its own books, including the half contributed by the sperm. If pages are torn, problems may start to show. Some embryos slow down or stop growing.
Days 5 to 6. A healthy embryo becomes a blastocyst, with two parts: an inner group of cells that becomes the baby, and an outer layer that becomes the placenta.
Days 6 to 7. The embryo attaches to the lining of the uterus. This is implantation.
How much comes from the egg, and how much from the sperm?
No one can give a true percentage split, because egg and sperm work together. A healthy egg can repair some torn pages in sperm DNA, so the same sperm might lead to different results with different eggs.
What research does show is where chromosome errors tend to come from. In one study of 967 IVF embryos, about 90% of whole chromosome errors came from the egg, while about 70% of partial chromosome errors came from the sperm (study). Even with young eggs, the egg leads. In embryos made from donor eggs, with donors averaging 24 years old, about 88% of chromosome errors traced back to the egg (study).
Part of the reason is how each cell is made. Research suggests about 20 to 30% of eggs in women and people assigned female at birth (AFAB) of reproductive age have the wrong number of chromosomes, compared with about 1 to 8% of sperm (review). The share of affected eggs tends to rise with age.
In plain terms: about 9 out of 10 missing or extra books come from the egg. But when a chapter is missing or extra, the sperm is more often the source.
Torn Pages: sperm DNA fragmentation
Sperm DNA fragmentation means breaks in the DNA strands inside sperm. A healthy egg can repair some of this damage, but not all of it. Heavy damage is harder to repair, and imperfect repair may lead to a missing or rearranged chapter.
Research is starting to connect these dots. A 2026 study found higher sperm DNA fragmentation was linked to more partial chromosome errors in embryos, especially in women and AFAB under 38, while the age remained the main factor for whole chromosome errors (study). An earlier study found a similar link (study). These studies show an association, which isn't the same as proof of cause.
Damaged pages can also affect an embryo that has the right number of books. Because the embryo starts reading its own genes early, sperm DNA damage may affect embryo development and contribute to implantation failure (review).
How this plays out with different paths to pregnancy
Timed intercourse. Sperm has to travel a long way and enter the egg on its own. Sperm count and how well sperm swim matter a lot here. DNA quality and the egg's quality still shape what happens after fertilization.
IUI (intrauterine insemination). Washed sperm is placed directly in the uterus, which shortens the trip. Sperm still has to swim to the egg and get in on its own, so clinics look closely at how many moving sperm are in the washed sample. When numbers are low, your doctor may suggest IVF instead.
IVF and ICSI. In standard IVF, sperm and eggs meet in a lab dish. In ICSI, a lab scientist places one sperm directly into each egg. ICSI helps get around low count and poor swimming. What it can't change is what's inside the sperm, including torn pages or a missing or extra book.
FET (frozen embryo transfer). The embryo's makeup is set before freezing, so egg and sperm have already done their part. If embryos were tested with PGT (genetic testing of embryos before transfer), you'll have some information about chromosome counts. Keep in mind that a biopsy samples only a few cells, and results can be complicated. For example, a 2026 study found partial chromosome errors are often mosaic, meaning only some cells are affected (study). Your fertility doctor and a genetic counselor can help you understand your specific results.
Pregnancy loss: how egg and sperm may play a role
Pregnancy loss is heartbreaking, and it's rarely anyone's fault. Many losses happen because an embryo has chromosome errors that don't allow it to develop, and most of these come from the egg. Studies of miscarriage tissue suggest the father contributes less than 10% of these chromosome count errors (study).
Sperm may contribute to pregnancy loss in a few ways:
Torn pages. A 2019 review of 13 studies found male partners of women with recurrent pregnancy loss had higher sperm DNA fragmentation than partners of women without losses (review). The evidence isn't fully consistent, though. A later study of 1,485 men found no meaningful differences in semen measures or DNA fragmentation between men in couples with and without recurrent loss (study).
A missing or extra book. Less often, sperm carries the wrong number of chromosomes. The egg can't correct this.
Rearranged books. Either parent can be healthy while carrying chromosomes with swapped pieces, called a balanced translocation. Their embryos may inherit an unbalanced version with missing or extra chapters, which can lead to miscarriage. Guidelines differ on when to test parents for this, and a 2026 ASRM committee opinion notes standard testing can miss small rearrangements (ASRM 2026).
Placenta genes. Some genes only work when they come from the father, and many of them help build the outer layer of the embryo that becomes the placenta. That's the part that attaches to the uterus and connects to your blood supply. Researchers are still learning how much everyday sperm quality affects placental development and pregnancy outcomes in people.
Care guidelines are paying more attention to the male partner. The 2022 European guideline for recurrent pregnancy loss now recommends assessing the male partner's lifestyle and age, and suggests sperm DNA fragmentation testing can be considered for diagnosis, though no single test has been established as best (ESHRE 2022).
What this means for you
Fertility is a two-person story. If you're facing failed transfers or pregnancy loss, it's reasonable to ask your fertility doctor whether a full evaluation of the male partner makes sense for you. Questions you might bring up include:
Would a semen analysis or sperm DNA fragmentation test be helpful for us?
Should either of us have genetic testing for chromosome rearrangements?
Would ICSI or embryo testing (PGT) make sense in our situation?
Are there lifestyle factors for both partners worth looking at?
Many people come to us for support alongside their fertility care, for both partners. Our role is to work with your medical team, never in place of it.
Frequently asked questions
Does sperm affect embryo quality? Yes. Sperm supplies half the embryo's genetic material and helps start development. Sperm DNA damage may affect embryo growth after about day 3 and has been linked to partial chromosome errors.
Is egg quality or sperm quality more important for IVF? On average, egg quality has the larger effect, since most whole chromosome errors come from the egg and a woman's age is the strongest predictor of IVF success. Sperm still matters, especially sperm DNA quality.
Can a healthy egg repair damaged sperm? Partly. Eggs can repair some sperm DNA damage, but not all of it, and they can't correct a missing or extra chromosome.
Can sperm cause miscarriage? Sperm can contribute, through DNA damage, chromosome errors, or an inherited rearrangement. Research on DNA fragmentation and recurrent loss shows a link in many studies but not all.
Does ICSI solve sperm problems? ICSI helps with low sperm count and poor movement. It doesn't change the DNA inside the sperm.