Dog Pregnancy Calculator (Whelping Date)

Background.

Two bitches are mated on the same Saturday. One owner had the vet run progesterone tests and knows she ovulated the Thursday before; the other knows only the date of the mating. The first can circle three days on the calendar for whelping. The second needs to keep a fortnight clear, and nothing short of hormone testing will shrink it.

The difference comes from canine reproduction itself. A bitch may accept a male over several days of her season, her eggs are released immature and need a couple of days before they can be fertilised, and sperm survive in her reproductive tract for several days. A mating can therefore fall well before or after the moment the pregnancy is really timed from, so the interval from mating to whelping varies far more than the pregnancy does.

The Merck Veterinary Manual handles this by publishing a different gestation length for each event you might have a date for: 64 to 66 days from the LH surge (the first rise in progesterone above baseline), 62 to 64 days from ovulation, 56 to 58 days from the first day of diestrus seen on a vaginal smear, and 58 to 72 days from the first day she permitted breeding. These are one pregnancy measured from four starting points, and the due date can be no more precise than the starting point you choose. The familiar "63 days" is the ovulation figure.

Alongside the due date you get a timetable of what to do and when: the first day a relaxin blood test is reliable, the best window for an ultrasound scan, and the day from which an X-ray can count the puppies, each moved onto your dating method. The result is an educational estimate, not veterinary advice, and it cannot confirm a pregnancy. If she goes past the end of her window, strains for more than 30 minutes without producing a puppy, goes more than 2 hours between puppies or has green or foul-smelling discharge before the first, call your veterinarian at once.

What is dog pregnancy calculator?

Canine gestation is the interval from the start of pregnancy to whelping, about nine weeks, but the number of days depends on the event it is counted from. The LH surge is the hormone signal that triggers ovulation and coincides with progesterone first rising above its baseline. Ovulation follows about two days later, which is why Merck's ovulation figure (63 days) is two days shorter than its LH figure (65). Diestrus, the phase after the fertile period, is recognised on vaginal cytology as the day the smear abruptly stops being dominated by cornified cells; it arrives roughly six days after ovulation, so its figure is 57 days. The first day she permitted breeding is the only anchor that needs no testing, and the least precise.

How to use this calculator.

1. 01Under What date do you know?, pick the event you actually have a date for. Progesterone results from your vet give the LH surge or ovulation; a cytology report gives the first day of diestrus. With no testing, choose the first day she permitted breeding — the first mating, not the last.
2. 02Enter the Date of that event. It is the hormone, cytology or mating date, not the day you noticed she was rounder or off her food.
3. 03Set Today’s date to the real date, since the field starts on a fixed example date. Change it to a future day, such as the day of a booked scan, to see where she will be then; it cannot be earlier than the event date.
4. 04Read the whelping window before the single date. On the three hormone-timed anchors the window is three days; on the mating anchor it is fifteen, and the date shown is simply its middle.
5. 05Use the ultrasound dates to book the confirmation scan and the radiograph date if you want a puppy count. Once the window opens, take her rectal temperature twice a day.
6. 06Read the stage line for what is useful now, and the note beneath it for the signs that mean phoning the vet rather than waiting.

The formula.

The due date is calendar arithmetic: whelping date = event date + L, where L is the gestation length for the event you chose, and the window is the event date plus the low and high ends of Merck's range for that event.

LH surge: L = 65, window 64–66 days. Ovulation: L = 63, window 62–64 days. Diestrual shift: L = 57, window 56–58 days. First day of breeding: window 58–72 days. Merck gives no single figure here, so the date shown is the midpoint, (58 + 72) ÷ 2 = 65 days, and it is labelled as a midpoint wherever it appears.

The test dates come from Merck's pregnancy-diagnosis guidance: relaxin reliable after 22–27 days, ultrasound best at 25–35 days, and radiography for litter size after 55 days (so from day 56). Those figures are quoted on the scale where whelping falls at 64–66 days, the LH-surge scale. To place them on your anchor, each is moved earlier by 65 − L days: no shift for the LH surge, 2 days for ovulation and 8 for the diestrual shift. On the ovulation anchor that puts relaxin from day 20, ultrasound at days 23–33 and the X-ray from day 54.

The mating anchor uses no shift, which treats the mating as if it fell on the LH surge. It may have fallen up to about a week either side, judging by the 58–72 day spread, so on that anchor the test dates carry the same uncertainty as the due date.

Gestation day is the number of whole days since the event date, and days to go is L minus that, negative once the single date has passed. The stage line compares the gestation day with each milestone in turn: too early, relaxin valid, ultrasound window, mid-to-late gestation, radiograph useful, whelping window open, and past the window. An event date more than 400 days before today is rejected, because one of the two dates must be wrong.

A worked example.

Frequently asked questions.

References& sources.

1. [1]MSD/Merck Veterinary Manual, "Whelping and Queening in Bitches and Queens" (Autumn P. Davidson, DVM, MS, DACVIM; last reviewed June 2025) — "Normal gestation in bitches is 56–58 days from the first day of diestrus, 64–66 days from the initial rise in progesterone from baseline, and 58–72 days from the first instance that the female permitted breeding"; pre-partum temperature drop to a mean of 37.1 °C, 8–24 hours before whelping; stage I labour within 24 hours of progesterone < 2 ng/mL.
2. [2]MSD/Merck Veterinary Manual, "Breeding Management of Bitches" (Davidson; last reviewed July 2025) — "Parturition terminates pregnancy 64–66 days after the LH peak"; gestation is 58–72 days from breeding at an unknown stage of estrus and 62–64 days from the day of ovulation; the diestrual shift permits due-date calculation 56–58 days later.
3. [3]MSD/Merck Veterinary Manual, "Pregnancy Determination in Bitches and Queens" (Davidson; last reviewed January 2026) — relaxin assay "specific and sensitive ... in bitches after 22–27 days gestation"; ultrasonography "best performed at 25–35 days gestation"; "Late gestational radiography (> 55 days) is the best method to determine litter size."
4. [4]MSD/Merck Veterinary Manual, table "Approximate Gestation Periods" — dog gestation is 58–72 days from breeding at an unknown stage of estrus, and 62–64 days from the day of ovulation.
5. [5]American Kennel Club, "The Reproductive Cycle in Dogs: From Breeding to Birthing" (Harriet Meyers; updated 18 December 2025) — "The normal gestation period in dogs is approximately 63 days as measured from ovulation", 65–66 days from the LH surge, and 58–72 days from the first time the bitch allowed breeding.
6. [6]Okkens AC, Teunissen JM, van Osch W, van den Brom WE, Dieleman SJ, Kooistra HS (2001). "Influence of litter size and breed on the duration of gestation in dogs." Journal of Reproduction and Fertility Supplement 57:193–197 (PMID 11787149) — 113 bitches of six breeds, each mated once on a progesterone-determined optimal day: gestation 58–65 days, mean 61.4 ± 1.5 days, negatively correlated with litter size; West Highland white terrier 62.8 ± 1.2 days vs German shepherd 60.4 ± 1.7 days.