Updated by the Progyny Clinical Team — August 2025.
Early pregnancy can be an emotional time. Every ache, pain, or symptom may make you wonder if it’s “normal.” For better or worse, normal can look very different from person to person, and most symptoms don’t predict what will happen next. Unfortunately, pregnancy loss is not uncommon. About 20% of pregnancies end in miscarriage, though the risk depends on your age and medical history.
A major cause of pregnancy loss is chromosomal abnormalities. In these cases, the pregnancy tries to grow but doesn’t have the right genetic material to continue. Some abnormalities still allow growth but come with serious health risks.
How chromosomes play a role
Chromosomes contain your genes. They sit inside the nucleus of every cell. Each person typically has 23 pairs (46 total) — one set inherited from the egg and one set from the sperm.
When the body makes eggs or sperm, it goes through a process that divides the chromosomes in half so that only one set is passed down. This process requires precise coordination. Errors sometimes occur, leading to an abnormal number of chromosomes. This is called aneuploidy, and it’s the most common cause of miscarriage.
People with ovaries are born with all the eggs they will ever have. Eggs sit in the ovaries for years before they finish dividing. As they age, the machinery that helps chromosomes separate becomes more prone to errors. This is why older eggs are linked to higher rates of aneuploid embryos and pregnancy loss. Sperm, by contrast, is made continuously throughout life, so it is less likely to have these errors, even with age.
Some examples of aneuploidy include:
- Monosomy: A chromosome (or part of one) is missing. Turner syndrome, when there is only one X chromosome, is the only type that can rarely result in a live birth.
- Trisomy: A chromosome (or part of one) is duplicated. The most well-known example is Down syndrome, caused by an extra copy of chromosome 21.
Structural chromosome abnormalities
Not all abnormalities are related to age. Some are structural changes that happen when chromosomes rearrange before being passed down. These can occur spontaneously (called de novo) or be inherited.
Types include:
- Translocations: A piece of one chromosome breaks off and attaches to another.
- Reciprocal translocation is when two chromosomes swap segments.
- Robertsonian translocation is when two chromosomes lose sections and fuse together.
These rearrangements can sometimes preserve all the necessary genetic material. But in many cases, they result in eggs or sperm that are missing or have extra DNA. This can cause miscarriage.
Structural chromosome abnormalities are less common than aneuploidy but may be considered if you’ve had multiple pregnancy losses.
What testing can be done?
There are a couple of tests that can be done after pregnancy loss:
- A test on pregnancy tissue that looks for missing or extra DNA across all chromosomes (microarray).
- A test on you or your partner that examines the number and structure of chromosomes (karyotype).
Key takeaways
Most pregnancy losses are due to random chromosomal abnormalities, and they are not your fault.
Age-related changes in eggs are the most common cause of aneuploidy, but other structural changes can also play a role. Testing may help identify whether chromosomal abnormalities were involved and guide future treatment decisions.
If you’ve had a miscarriage, you’re not alone. Your fertility care team can help explain your options and whether testing may be helpful.
Progyny is here for you, too. Please contact your Progyny Care Advocate for support.
Disclaimer: The information provided by Progyny is for educational purposes only and is not medical advice. Always consult a qualified healthcare provider for medical guidance.