Karyotype

A karyotype is a genetic test examining chromosome number and structure from a blood sample. It is most often performed for recurrent miscarriages, infertility and suspected chromosomal disorders; results are interpreted with a doctor and, if abnormal, with genetic counselling.

Who the test is for

• couples with recurrent miscarriages, following medical assessment;
• men with no sperm or a very low sperm count in the ejaculate;
• women with premature ovarian failure before age 40 or no first period;
• couples before assisted reproduction when recommended by a doctor;
• children and adults with suspected chromosomal disorder due to developmental delay, congenital anomalies or short stature;
• girls with absent puberty;
• family members when a chromosomal change is known.

What a karyotype is and what it shows

Human cells with a nucleus, apart from eggs and sperm, usually have 46 chromosomes: 22 pairs of autosomes and one pair of sex chromosomes. Women have two X chromosomes, men one X and one Y, so a normal result is written 46,XX or 46,XY. White blood cells from the sample are cultured in the laboratory and their chromosomes examined microscopically. An image of the ordered chromosomes of one cell is a karyogram; a karyotype describes the person’s entire chromosome set.

The test detects two types of change:

• chromosome number changes (aneuploidies), with an extra or missing chromosome, for example Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), Patau syndrome (trisomy 13), Klinefelter syndrome (47,XXY) or Turner syndrome (45,X);
• chromosome structure changes, such as translocations, where parts of two chromosomes exchange places, and inversions, deletions and duplications.

In a balanced translocation no genetic material is missing or extra, so carriers are usually healthy and unaware. Nevertheless, it can cause conception difficulties, more miscarriages or a child with an unbalanced chromosome set. Karyotyping also detects mosaicism, where a person has cells with different chromosome numbers.

A carrier of a balanced structural chromosome change is found in around two to four percent of couples with recurrent miscarriage. The European Society of Human Reproduction and Embryology (ESHRE) does not recommend parental karyotyping for all such couples, but following medical assessment, for example when miscarriage tissue shows an unbalanced chromosome change or there are children with congenital anomalies in the family. In men with no sperm or a very low sperm count, the most common chromosomal cause is Klinefelter syndrome (more in Causes of male infertility); in women karyotyping is usually performed alongside hormone tests such as FSH and AMH.

Preparation and sample

Blood is taken from a vein. No fasting or special preparation is needed. Mention recent transfusion or chemotherapy during collection because these may affect the test. When a doctor recommends parental karyotyping, both partners usually have testing, each from their own blood sample.

How to interpret the result

A normal result (46,XX or 46,XY) means chromosome number and microscopically visible structure are normal. Karyotyping does not detect very small genetic material losses or gains, single-gene changes such as cystic fibrosis, or Y chromosome microdeletions in infertile men; other genetic tests detect these. Normal parental karyotypes do not rule out other miscarriage causes: around half of early miscarriages result from a random embryo chromosome error rather than a parental change.

A doctor interprets an abnormal result, with free genetic counselling also available. Counselling explains what the change means for you and future pregnancies and options such as prenatal diagnosis or embryo genetic testing during assisted reproduction.

How to arrange testing

You can have blood taken without an appointment or referral at one of our branches across Croatia (listed under Contact). If testing with your partner, mention this during collection. Prices for all tests are in the service price list.

Frequently asked questions

Should both partners have a karyotype?

Not always. Karyotyping is not routine for all couples with infertility or recurrent miscarriage. If a doctor recommends parental karyotyping based on results and history, for example recurrent miscarriages, they may recommend both partners because either can carry a change. Each provides a separate sample and receives a separate report.

Does karyotyping need repeating?

Generally not. A karyotype does not change during life, so one result is sufficient unless a doctor requests repetition for a specific reason.

Can a balanced translocation carrier have a healthy child?

Yes. Miscarriage risk is higher than in other couples, but most carriers also have healthy children. Options for another pregnancy are discussed in genetic counselling.

Is a karyotype the same as a genetic test?

Karyotyping is one type of genetic test. It shows chromosome number and major structural changes, but not individual-gene changes, for which other tests exist.