Mismating: Emergency Contraception in 72 Hours

A queen that slipped out and mated gives you about 72 hours before the options narrow to one. Emergency contraception in cats means a vet visit, not a pill from the cabinet: aglepristone, two injections 24 hours apart, is the standard medical route, and spaying is the only permanent one. Human hormones sold for this purpose have killed cats and must stay out of the plan.

Why 72 hours is the real deadline

Cats ovulate 24 to 48 hours after mating, and the eggs stay viable for roughly 12 to 24 hours after release. Aglepristone blocks progesterone at the uterine receptor, so it can only interrupt a pregnancy that is still settling in, which is why the licensed window is the first 72 hours and why day 1 or day 2 gives the best result. By day 5 the embryo is implanted and success drops sharply. A mismating that never triggered ovulation is a different story: no corpus luteum means no pregnancy to stop, and the queen simply comes back into heat in 2 to 3 weeks.

Aglepristone: dose, timing, what to expect

The protocol most vets use is 10 mg/kg under the skin, repeated once after 24 hours, with the first dose inside 72 hours of mating and ideally inside 24. It is licensed for dogs in the EU and UK and used off-label in cats, so the queen needs a physical exam, an accurate weight, and a conversation about uterine infection risk before the first injection. Expect mild swelling or tenderness at the site for 1 to 2 days, possible sleepiness, appetite down for 24 to 48 hours, and a little vulvar discharge as progesterone falls. Some queens show mammary enlargement that resolves in 7 to 10 days. It is not for a queen with pyometra, diabetes, or a known uterine infection, and it does not cover a second mating later in the same cycle.

What never to use

Diethylstilbestrol and other human estrogens were used decades ago and are now a hard no. Estrogen in a cat causes bone marrow suppression, sometimes irreversible aplastic anemia, plus cystic endometrial hyperplasia and pyometra; the dose that prevents pregnancy sits uncomfortably close to the dose that kills. Progesterone injections such as medroxyprogesterone suppress heat in some countries but raise the risk of pyometra, diabetes, and mammary tumors, so they are not an emergency answer. Misoprostol and dinoprost can empty a uterus but cause vomiting, cramping, and, in the wrong case, uterine rupture; they act later in pregnancy and belong in a clinic with monitoring. None of these should be given at home or ordered online.

Time window, option, and risk

Time since mating	Option	Main risk or limit
0 to 24 hours	Aglepristone 10 mg/kg, second dose at 24 hours	Injection-site reaction, 1 to 2 days of poor appetite
24 to 72 hours	Same protocol, slightly lower success	Failure rate climbs after 48 hours
Day 4 to 20	Spay, or wait and scan at day 21 to 25	Implantation may be complete; medication is off the table
Day 21 to 40	Spay-abort, or plan for the litter	Routine surgery, but the uterus is more vascular
After day 45	Spay-abort or a planned caesarean	Late termination equals a delivery and its risks
Any time	Ovariohysterectomy	Ends any future pyometra risk as well

Spaying is the only certain answer

An ovariohysterectomy at 3 to 4 weeks of gestation takes about 10 minutes longer than a routine spay and costs maybe 20 to 40 percent more at most clinics. Recovery is the same: 10 to 14 days of no jumping, a cone or recovery suit for 7 days, and a skin check at 10 to 14 days. Queens spayed before their first heat have a far lower rate of mammary carcinoma than intact queens, and the pyometra risk leaves with the uterus. If she is a planned breeding cat, spaying ends that plan, so the decision has to be made in hours rather than weeks.

If the window has already closed

Do nothing heroic. Count 21 days from the mating date and book an ultrasound: by day 25 a heartbeat is visible and the answer is clear. A relaxin test from day 25 confirms pregnancy with high accuracy, and an X-ray at day 45 counts skeletons if you want a number before birth. From there the choice is spay-abort or prepare: a quiet room, a nesting box, and a birth plan for day 63 to 65. Watch for discharge, fever, or depression at any point, because a mismating that ends in pregnancy can also end in infection.

First hour checklist

1. Write down the date and time of mating, and whether intromission was actually seen.
2. Call the clinic and say the words mismating and 72 hours so the appointment is triaged correctly.
3. Do not bathe her and do not give any human medication while you wait.
4. Keep her indoors, away from intact males; she can mate again within 2 to 3 weeks.
5. Ask the price of both aglepristone and spay before choosing, since the second may be needed anyway.

If aglepristone is used, the follow-up scan at day 30 is not optional, and a uterine infection is the main thing to catch early, so read pyometra signs before you need them. If a pregnancy does establish, the week-by-week checkpoints are in our cat pregnancy timeline. For the recovery period after surgery, see when to spay or neuter.

Frequently asked