Foaling date calculator: estimate when your foal will be born

How the foaling date is worked out

The calculator counts 340 calendar days from the date you give it. That is the average gestation length of a mare, roughly eleven months. The same figure drives the gestation tracking of registered Equ'un users, so the public tool and the platform cannot show two different dates for one and the same breeding.

The starting point of the count is called the anchor. The equine convention counts day zero from ovulation, meaning the moment the ovary releases the egg cell, because that is the real biological start of the gestation. In natural mating, ovulation follows the covering by one to two days, sometimes more. That gap looks negligible, and it is not: the window for detecting twins closes on the sixteenth day, and two days of drift are enough to miss it.

If all you know is your breeding date, leave the default option. The result stays perfectly usable, simply a little less precise. If your veterinarian has confirmed ovulation by ultrasound or by palpation, choose the second option: every milestone then realigns on the true day zero.

The calculation only handles calendar days, never hours. There is therefore no time-zone question, and a leap year crossed along the way is taken into account on its own.

Why a birth window, and not a single date

Two mares covered on the same day can foal seven weeks apart. The expected foaling date is therefore a point estimate, useful for organising your watch, and not a deadline nature would keep. The normal birth window is the real information: it says from when a birth is expected, and until when it stays ordinary.

Four factors make the length vary from one gestation to the next:

• The season of the covering. Mares covered in winter generally carry a little longer than mares covered in spring. Lengthening days appear to speed up the end of the gestation.
• The sex of the foal. Colts, meaning male foals, are born on average a few days later than fillies.
• The size of the mare. Pony mares often foal a little earlier than riding-horse or draught mares.
• The age and the gestation rank. The length varies with the age of the mare, and with whether she is foaling for the first time or not.

A given mare also tends to repeat a similar gestation length from one year to the next. If you have noted how long her previous gestations lasted, that will tell you more than any population average.

The thresholds, and the vocabulary that keeps them apart

The calculator shows four markers, and it names them precisely, because they are different things. The start of the normal foaling window falls at 320 days, the expected foaling date at 340 days, and the end of the normal foaling window at 362 days. Those three values are the ones of the Merck Veterinary Manual, the international veterinary reference, and they are exactly the ones the platform already uses for the gestation tracking of its registered users.

The fourth marker is of another nature. At 370 days, the tool shows an overdue gestation and recommends veterinary advice. This is not the upper bound of a normal window, it is the length beyond which a gestation that drags on warrants an examination, in particular because of the risk of dystocia, meaning a difficult birth: a foal left too long in the uterus can grow too large to be delivered without help. Conversely, some foals from a very long gestation are born dysmature, meaning well developed in appearance, but weak and lacking muscle tone.

You will read elsewhere that the normal range runs from 320 to 370 days, and this is not a contradiction. The veterinary literature does not agree on a single upper bound: some authors keep 320 to 362 days, others 320 to 367, others again 310 to 380. Rather than pick one range against another, Equ'un has separated the two ideas that those figures mix together. The normal foaling window stops at 362 days, because that is the point where watching more closely becomes reasonable. And 370 days stays on screen, under its real name: an overdue threshold, meaning the moment to call the veterinarian, and not the end of a comfortable stretch. A birth between 362 and 370 days is in no way abnormal in itself, it simply deserves to be followed. You therefore find in the tool the two figures you have come across elsewhere, each in its right place.

In the other direction, a foal born before 320 days is premature. Its lungs are insufficiently developed, it is weak and can struggle to stand and to suckle, which makes hospitalisation in an equine clinic almost systematic. That is why the start of the normal window is a marker in its own right, and not merely the left edge of a bar.

A worked example: a mare covered on 4 May 2026

Take a mare covered on 4 May 2026, with no confirmed ovulation date. The count therefore starts from the breeding date. The expected foaling date falls on 9 April 2027, and the normal birth window runs from 20 March 2027 to 1 May 2027. The overdue threshold, for its part, is reached on 9 May 2027. Here is the full calendar the calculator draws from it.

The first four rows are the ultrasound scans. Twin detection is played out between 18 and 20 May 2026, and it is the most urgent entry of the calendar: past the sixteenth day, a surplus embryo can no longer be removed without compromising the other one. Then come the three injections against equine herpesvirus, also called rhinopneumonitis, a virus whose main danger in a pregnant mare is late abortion, often with no warning sign at all. Here they fall on 1 October 2026, 30 November 2026 and 29 January 2027.

Had the veterinarian confirmed an ovulation on 6 May 2026 rather than the covering of 4 May, every one of these dates would shift by two days, without exception: foaling would then be expected on 11 April 2027. That is precisely what the second option of the form changes.

What this calculator does not tell you

It does not confirm a gestation. A breeding date is not proof: only an ultrasound scan carried out by a veterinarian says whether the mare is in foal. The calculator assumes that the gestation really did start on the day given, and it has no way of checking that.

It replaces no appointment. The dates it sets down are markers for organising yourself, not prescriptions. The vaccination protocol, the number of scans and their timing are decided with your veterinarian, according to the mare, her history and the health situation of the stud.

It does not predict the night of the foaling. The signs that truly announce it are read on the mare and not on a calendar: the udder develops two to six weeks beforehand, the teats fill with colostrum, meaning the first milk, which is very rich in antibodies, the pelvic ligaments slacken a few days before, and wax forms at the tip of the teats in the 24 to 48 hours that precede it.

Frequently asked questions

Useful sources

• Équipédia (IFCE, the French horse and riding institute), managing the breeding mare
• Merck Veterinary Manual, management of the pregnant mare
• Équipédia (IFCE), vaccination against equine herpesvirus