Estimate the whelping date and window from the date you know, with veterinary ranges
Updated October 2026
A bitch may accept breeding over several days around ovulation, so the breeding date is a poor clock. Veterinary references count from hormonal events instead. The Merck Veterinary Manual gives 64-66 days from the luteinizing hormone (LH) surge or the first rise in progesterone above about 2 ng/mL, 56-58 days from the first day of diestrus (seen on vaginal cytology), and 58-72 days from the first day she accepted breeding. Cornell’s Riney Canine Health Center puts it at 63 days from progesterone-timed ovulation, plus or minus 1-2 days, versus plus or minus 7 days from the first breeding.
Pick the date you know: ovulation or the LH surge (from your vet’s progesterone or LH testing), the first breeding, or the first day of diestrus. Then enter the date. For a cat, choose the mating option: queens give birth 64-66 days after the LH surge that mating triggers. The result shows the most likely date, the window, the rule used, when ultrasound measurements are most predictive, when to start taking temperatures, and when to call the vet.
The calculator adds the published number of days to your date. From ovulation on March 1, 2027: March 1 + 63 days = May 3, 2027, with a window of May 1 to May 5 (61 to 65 days). From a first breeding on the same day: March 1 + 58 = April 28 to March 1 + 72 = May 12, with May 3 as the midpoint. A research review of predicting whelping dates (Siena and Milani, 2021) found pregnancy length counted from breeding varies widely, and that the drop in progesterone just before whelping is the most accurate predictor.
| What you know | Most likely date | Window | Window width |
|---|---|---|---|
| Progesterone-timed ovulation on March 1, 2027 | May 3, 2027 | May 1, 2027 to May 5, 2027 | 5 days |
| LH surge detected on March 1, 2027 | May 5, 2027 | May 4, 2027 to May 6, 2027 | 3 days |
| Bred twice, first on March 1, 2027 | May 3, 2027 | April 28, 2027 to May 12, 2027 | 15 days |
| Cat mated on March 1, 2027 | May 5, 2027 | May 4, 2027 to May 6, 2027 | 3 days |
Gestation: 63 ±1-2 days from ovulation (Cornell); 64-66 days from the LH surge, and 65 days in a Cornell ultrasound study of 83 bitches (Kutzler and colleagues, 2003). Ultrasound: in that study, fetal measurements taken about 30 days after the LH surge predicted the whelping date best, within 1 day in 75% of bitches and within 2 days in 87% after adjusting for body weight. Temperature: normal rectal temperature is 100-102.5°F; Cornell notes it transiently drops below 99°F within 24 hours of birth, though the drop is not always detected, and Merck puts it at 98.1-100°F 8-24 hours before whelping.
Book a veterinary check to confirm the pregnancy; Cornell notes ultrasound can be used about midway through gestation, and the Kutzler study found measurements around day 30 after the LH surge best for dating. Ask your vet what care and diet she needs for the rest of the pregnancy. Before the window opens, set up a quiet whelping area, start taking her rectal temperature twice a day and keep a log, and keep your vet’s and an emergency clinic’s numbers handy.
Call your veterinarian or an emergency clinic right away if more than two hours pass between puppies, if she strains hard without producing a puppy, if she seems weak or in distress, or if the due window passes without labor. Cornell notes each puppy typically takes 0-30 minutes to be born once active labor starts. Cornell advises seeking care immediately for difficult birth (dystocia) or whenever you are unsure labor is progressing normally. This calculator gives dates, not a medical assessment of the pregnancy.
Counting only from the breeding date and then worrying when puppies arrive a week "early" or "late". Counting from the last of several breedings: use the first. Assuming a positive breeding means pregnancy: confirm it with your vet. Missing the temperature drop by checking only once a day. Using a human pregnancy calculator, which counts 280 days.
It cannot confirm a pregnancy, count puppies, or detect problems; it does not adjust for litter size or breed, and it does not replace progesterone testing or ultrasound. If you are unsure of any dates, your veterinarian can estimate gestational age from ultrasound measurements.