Sexing Kittens: The 3-5 mm vs 8-12 mm Test

Sexing a kitten is a measurement, not an opinion. Between the anus and the genital opening sits a strip of skin, and the length of that strip separates the sexes: 3–5 mm in a female, 8–12 mm in a male. By day 7 the difference is wide enough to record on a litter sheet with confidence, and by 3 weeks it is obvious to anyone. The shape of the opening helps confirm the call, but it is supporting evidence, never the primary test.

How to take the measurement

Work in a warm room with a lamp angled across the perineum, because flat overhead light hides the shadow that makes the gap readable. Hold the kitten chest-down in your palm with the head toward your wrist, lift the tail gently with your thumb, and look straight down at the two openings. Do not stretch the tail up; tension flattens the skin and shortens the apparent distance.

Measure with a steel rule laid alongside the gap, or use a reference you always carry: the width of a fingernail is about 10 mm, so a gap narrower than half a nail is female and one that covers most of the nail is male. Photograph the litter at day 7 with the rule in frame. That photo settles arguments at 8 weeks, when chunky kittens and long fur start hiding anatomy.

What changes week by week

Age	Female	Male
Birth to 3 days	Slit 3–5 mm below anus, may look swollen from maternal hormones	Round opening 8–12 mm below anus; scrotum flat and empty
7 days	Gap stable, opening a vertical slit	Gap clearly double the female distance
3 weeks	Vulva a small slit close to the anus	Testicles palpable as two 3–4 mm ovoids
6 weeks	Perineum flat, no bulge	Scrotum visible as a rounded sac
8–12 weeks	Unchanged anatomy, fur may obscure it	Testicles 8–10 mm, easy to palpate in most males

Testes: when they arrive, and when they do not

Most tom kittens have both testicles in the scrotum by 3–4 weeks, and nearly all by 6–8 weeks. Until then the scrotum is a flat fold of skin and palpation proves nothing. A testis that has not descended by 12 weeks deserves a recheck at 16 weeks, and if it is still absent the kitten should be examined by a vet rather than sold as a stud prospect.

Cryptorchidism is uncommon but not rare, reported in roughly 1–2% of male cats, with the right testis retained more often than the left and unilateral retention far more common than bilateral. Some lines and some breeds carry more of it, and a retained testis is inherited, so a cryptorchid male should be neutered and never used at stud, even after surgical correction. Bilateral retention also raises the risk of testicular tumours later, which is one more reason the cat goes to the vet rather than the shows.

Three ways breeders get it wrong

1. Calling the sex at 2–3 days. Maternal hormones leave the female vulva puffy and the male scrotum swollen; both look ambiguous. Wait until day 7 and measure.
2. Trusting the shape alone. A fat kitten with a deep perineal fold can make a male look like a female, and a female in oestrus at 5 months can look open and round. The distance is the tie-breaker, every time.
3. Ignoring ambiguous anatomy. Intersex kittens exist, including XXY males and true hermaphrodites, and they present with a gap that sits between the two norms, sometimes with a small penis and a vulva-like opening. These cats need a veterinary exam and karyotype, not a guess on a sales contract.

Recording the call

Write sex next to birth weight on the litter sheet, and add the measured gap in millimetres for the first 10 days. When a kitten is reserved, put the sex and the date of the check in the contract, and tell the new owner that the 8-week vet visit is the second checkpoint. Breeders who keep this record catch their own errors early, because a male recorded at 4 mm on day 3 and 11 mm on day 10 tells you the day-3 call was noise.

If you are tracking the whole first year, the growth and behaviour markers on our kitten milestones timeline line up with these checks; sexing belongs in the same notebook as weight and eye opening. The daily weight and temperature routine in neonatal kitten monitoring is the right place to note any swelling, discharge or hernia around the genitals, and the anatomical terms are defined in the cat glossary if you are writing handover notes for a co-breeder.

When the answer is not clear

Call a vet for a kitten with a gap that falls between 5 and 8 mm at 4 weeks, a swelling that changes size, a persistent discharge, or an umbilical or inguinal hernia near the genital area. Also call if no testis can be felt by 16 weeks. None of these are emergencies in a 6-week-old, but all of them change the plan before the kitten is sold or kept.

Five steps to get it right

• Measure at day 7, not day 2, and write the number in millimetres.
• Use one light source angled from the side and lift the tail without stretching.
• Confirm the call at 3 weeks by palpating for two 3–4 mm testicles.
• Recheck any male at 12 and 16 weeks; refer if the testis is absent.
• Never breed a cryptorchid, and note the defect in your records for the line.

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