Use the breeding date to estimate your guinea pig's likely birth window, middle due date, and how far along the pregnancy may be today.
How to use our Guinea Pig Pregnancy Calculator
- Enter the Breeding date as the date mating happened, or the best date you know.
- Choose Date type: pick Exact mating date if you know the day breeding happened, or First day together if the female was placed with a male and mating could have happened later.
- Click Calculate to see the Likely birth window, Estimated due date, Pregnancy day today, and the days remaining.
- Sanity-check the result: if you picked First day together, the window should be wider and the Assumption note should explain why.
- Use the Status note to see whether the pregnancy looks early, mid, near the likely window, or past the likely window.
Definitions
Breeding date: The date you are using as the start point for the estimate.
Date type: Tells the calculator whether that date was the exact mating day or only the first day the female was with a male.
Likely birth window: The earliest and latest dates birth is most likely to happen based on the chosen date type.
Estimated due date: The middle date of the estimate. It is useful for planning, but it is not exact.
Pregnancy day today: How many days have passed since the entered breeding date. If the date is today, this value is 0.
Days until likely window starts: Days from today until the earliest likely birth date. A negative value means that day has already passed.
Assumption note: A short explanation of how the calculator interpreted your entered date.
Common mistakes and quick fixes
Mistake: Entering a future Breeding date .
Fix: Use today or an earlier real date. The calculator blocks future dates because Pregnancy day today cannot be negative.
Mistake: Choosing Exact mating date when you only know when the guinea pigs were first housed together.
Fix: Change Date type to First day together so the Likely birth window and Estimated due date allow for later mating.
Mistake: Treating the Estimated due date as a guaranteed birth day.
Fix: Use Likely birth window as the main answer. The due date is only the middle estimate.
Mistake: Thinking a negative Days until likely window starts or Days until estimated due date means the calculator is broken.
Fix: A negative number simply means that date has already passed.
Mistake: Assuming Pregnancy day today is exact even when the breeding date is only a guess.
Fix: Treat it as an estimate based on the date you entered, and read the Assumption note for context.
Limitations & Key Assumptions / Boundary Conditions
- The result is an estimate based only on Breeding date and Date type. It does not use exam findings, ultrasound, weight change, or litter details.
- If you choose First day together, the calculator assumes mating could have happened later, so the birth window is widened on purpose.
- The Estimated due date is a planning midpoint, not a guaranteed delivery date.
- If the entered date is only a rough guess, every output, including Pregnancy day today, is only a rough estimate.
- Very old dates still calculate, but a passed window does not confirm what happened. It only shows that the likely dates are already behind today.
- Results use calendar-day arithmetic, so leap years and month-end changes are handled by date math rather than fixed month lengths.
Methodology
How the calculator works
The calculator starts with your Breeding date and then adds a set number of days to estimate the likely birth dates. The main answer is the Likely birth window, because a range is more realistic than one exact day.
Formulas used
earliest date = breeding date + 59 days
latest date = breeding date + 72 days
estimated due date = breeding date + 68 days
Those three formulas are used when Date type is Exact mating date.
earliest date = breeding date + 59 days
latest date = breeding date + 89 days
estimated due date = breeding date + 76.5 days
Those formulas are used when Date type is First day together. The later end of the window is widened by 17 days to reflect that mating may not have happened on the first day together.
pregnancy day today = days between today and breeding date
days until target date = days between target date and today
A negative days-until value is kept as negative. That means the target date has already passed.
Mini example
If the Breeding date is January 1 and Date type is Exact mating date, the calculator adds 59 days for the early end, 68 days for the midpoint, and 72 days for the late end. That gives a Likely birth window of March 1 to March 14 and an Estimated due date of March 10 in a non-leap year.
How to read the status note
The Status note compares today with the likely window. It labels the pregnancy as earlier in the timeline, closer to the likely window, inside the likely window, or past it so you can quickly tell where today falls.
Assumptions behind the math
The calculator assumes standard calendar dates, one breeding timeline, and the fixed day values listed above. It does not adjust for the female's health, exam findings, uncertainty beyond the chosen Date type, or uncommon cases where birth happens outside the usual range.