Important: Not medical advice. This report is a general estimate from a published formula. It is not medical advice, a diagnosis, or a treatment plan, and it doesn't account for your health history, medications, pregnancy or other conditions. Talk to a doctor, midwife or registered dietitian before making health decisions based on it. Use of this report is at your own risk; see aiocalc.com/disclaimer and aiocalc.com/terms.

Health

Pregnancy Calculator

Track your pregnancy week by week, find out when to test, or check a dog or cat's due date. Enter your usual cycle length for a closer estimate.

Updated October 11, 2026 · 4 cited sources

Your dates

WeeklyWhen to TestDog/Cat

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Due date: —

Enter the first day of your last period.

This trimester

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Your pregnancy timeline

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Estimate only, not medical advice — always confirm dating and testing timing with your doctor, midwife, fertility clinic, or vet.

Pregnancy week by week

Pregnancy is measured in weeks from the first day of your last menstrual period (LMP), not from conception — a full-term pregnancy runs about 40 weeks from LMP, or roughly 38 weeks from actual conception. Enter your LMP above to see your current week, day, and estimated due date, updated live as you change the date.

The 40-week count assumes a 28-day cycle with ovulation around day 14. If your cycle is usually longer or shorter, enter its length: the calculator moves your gestational age and due date by the difference. With an LMP of January 1, 2026, a 28-day cycle gives a due date of October 8; a 35-day cycle moves it a week later, to October 15, and you'd be a week "younger" on any given day. The due date calculator does the same from a conception or IVF transfer date, and the ovulation calculator works forward from your cycle if you're trying to conceive.

Trimesters by week

Following the American College of Obstetricians and Gynecologists, the calculator counts week 13 as first trimester and week 27 as second trimester. A new trimester starts on day 0 of week 14 and of week 28:

Pregnancy trimesters by gestational week
Trimester	Gestational age	Relative to a 40-week due date
First	Week 0 day 0 to week 13 day 6	From your LMP to about 26 weeks before the due date
Second	Week 14 day 0 to week 27 day 6	About 26 to 12 weeks before the due date
Third	Week 28 day 0 to birth	The last 12 weeks, through the due date and beyond

"Week 13 day 6" means 13 full weeks and 6 days have passed since your LMP, so you are in the 14th week. Apps differ on whether they say "13 weeks" or "week 14" for the same day, which is the most common reason two pregnancy trackers disagree by one.

When can I take a pregnancy test?

Home urine tests detect hCG, a hormone that rises after implantation — testing before your missed period (about 14 days post-ovulation) increases the risk of a false negative, since hCG may not be concentrated enough yet. Quantitative blood tests, usually done at a clinic, can detect hCG a few days earlier than a home urine test.

Testing after IUI or ovulation

If an hCG trigger shot was used as part of an IUI cycle, testing too early can produce a false positive from the leftover trigger medication rather than an actual pregnancy — waiting the full 14 days after IUI avoids this. Fertility clinics typically schedule a precise beta hCG blood test around this point rather than relying on a home test.

Dog & cat pregnancy

The calculator uses 63 days for dogs and 64 days for cats — both notably shorter than human pregnancy. The Merck Veterinary Manual lists approximate gestation as 62–64 days for dogs, but 58–72 days when counted from the first breeding the female accepted, and 64–66 days for cats counted from the hormone surge that mating triggers. Actual timing depends on exactly when ovulation occurred relative to mating, which can vary by several days, so treat this as an estimate and confirm with a vet for anything time-sensitive.

Sources

The formulas on this page come from the published research below. Last updated October 11, 2026. This calculator gives estimates for general information. It isn't medical advice, so check anything important with a qualified professional.

American College of Obstetricians and Gynecologists. Committee Opinion No. 700: Methods for Estimating the Due Date. Obstet Gynecol. 2017;129(5):e150–e154.Used for: Gestational age and week-by-week dating from the last menstrual period
Wilcox AJ, Baird DD, Weinberg CR. Time of implantation of the conceptus and loss of pregnancy. N Engl J Med. 1999;340(23):1796–1799.Used for: Implantation timing behind when a pregnancy test can turn positive
Merck Veterinary Manual. Approximate Gestation Periods.Used for: Dog and cat gestation lengths
Merck Veterinary Manual. Whelping and Queening.Used for: Dog and cat gestation ranges from breeding or LH surge

Frequently asked questions

Can a pregnancy test be positive before a missed period?

Sometimes. A test can only pick up hCG after the embryo implants, and in the Wilcox study cited below implantation happened 6 to 12 days after ovulation, most often on days 8 to 10. Sensitive home tests may turn positive a few days before a missed period, but a negative result that early doesn't rule pregnancy out. Retest after your period is due.

Why does my ultrasound show a different week than this calculator?

Period-based dating assumes you ovulated on a set day, while an early scan measures the embryo directly, so the two often differ by a few days. If the gap is large enough, your provider will move your due date to match the scan and keep it there. From then on, use the scan date: enter the new due date in the Due Date Calculator to get matching weeks.

What if my periods are irregular?

Enter your usual cycle length and the calculator shifts the dates to match, which helps if your cycles are long but steady. If they vary by more than a few days each month, no LMP-based estimate is reliable, and an early ultrasound is the better way to find out how far along you are. Tell your provider about irregular cycles at your first visit.

Should I count a dog's due date from the first or last mating?

Count from the first mating the female accepted, and treat the result as the middle of a wide window. The Merck Veterinary Manual gives 58 to 72 days from first accepted breeding, because sperm can survive for days and ovulation timing varies. Your vet can narrow the date with progesterone testing or ultrasound if an accurate whelping date matters.