Pregnancy & Fertility Calculators
4 tools in this collection — free, instant, and private in your browser.
Pregnancy is a journey measured in weeks, and keeping track of where you are in that journey — along with key upcoming milestones — can feel overwhelming without the right tools. This small but focused collection of calculators covers the most common date and timing questions that arise before conception, during pregnancy, and as the due date approaches. All calculations are based on standard clinical conventions used by midwives and obstetricians worldwide.
The starting point for many people is the Ovulation and Fertile Window Calculator. Conception can only occur during a narrow window each cycle when a released egg is viable — typically 12 to 24 hours — but because sperm can survive in the reproductive tract for up to five days, the fertile window spans roughly six days per cycle. Entering the first day of your last period and your average cycle length gives you an estimate of when ovulation is likely to occur and which days carry the highest probability of conception.
Once pregnant, the Pregnancy Due Date Calculator uses Naegele's Rule — the standard clinical formula — to estimate your estimated due date (EDD) from the first day of your last menstrual period. This date is 40 weeks (280 days) from that starting point. Keep in mind that only about 5 percent of babies are born exactly on their due date; it is better understood as the midpoint of a normal delivery window spanning roughly 37 to 42 weeks.
The Pregnancy Week Calculator tells you exactly how many weeks and days pregnant you are right now, and which trimester you are in. The Pregnancy Weeks Remaining Calculator shows how many weeks remain until your due date, useful for planning leave, preparing your home, and scheduling prenatal appointments. These calculators are intended as planning aids and do not replace guidance from a qualified healthcare provider, who may adjust your dates based on ultrasound measurements.
All pregnancy & fertility calculators
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| Tool | What it does |
|---|---|
| Ovulation & Fertile Window Calculator | Find your most fertile days and estimated ovulation date from your last period and cycle length. |
| Pregnancy Due Date Calculator | Calculate your estimated due date (EDD) from the first day of your last menstrual period (LMP). |
| Pregnancy Week Calculator | Find out how many weeks pregnant you are from the first day of your last menstrual period. |
| Pregnancy Weeks Remaining Calculator | Calculate how many weeks and days remain until your estimated due date. Enter your due date and today’s date. |
Frequently asked questions
- How is a pregnancy due date calculated?
- The standard method is Naegele's Rule: take the first day of your last menstrual period, add one year, subtract three months, and add seven days. This works out to approximately 280 days or 40 weeks from the start of your last period. The formula assumes a 28-day cycle with ovulation on day 14; if your cycle is significantly shorter or longer, your healthcare provider may adjust the due date. An early ultrasound (before 14 weeks) is the most accurate way to confirm or refine the estimated due date.
- When does each trimester start and end?
- The first trimester runs from week 1 through the end of week 12. The second trimester covers weeks 13 through 26, a period often described as the most comfortable phase of pregnancy. The third trimester runs from week 27 until delivery, typically around week 40. Some clinicians define trimester boundaries slightly differently, so your provider may use a variation of these ranges, but the 12/13 and 26/27 week breakpoints are the most widely used.
- Is the fertile window always the same days each cycle?
- No. The fertile window shifts depending on when ovulation occurs, which can vary from cycle to cycle — even in people with generally regular cycles. Factors like stress, illness, travel, and significant changes in weight or exercise can delay or advance ovulation. The Ovulation Calculator provides an estimate based on average cycle length, but for the highest accuracy, consider combining it with basal body temperature tracking or over-the-counter ovulation predictor kits, which detect the LH surge that precedes ovulation by 24 to 36 hours.