Pregnancy dating
- Due date from the last period: Naegele’s rule — first day of the last menstrual period + 280 days (40 weeks), adjusted by the difference between your cycle length and 28 days. Used by the Pregnancy Calculator and Due Date Calculator.
- From conception: conception date + 266 days (38 weeks).
- IVF: transfer date + 266 days − embryo age at transfer (day 3, 5 or 6), or egg retrieval date + 266 days.
- Ultrasound: scan date + (280 days − gestational age measured on the scan). Early (first-trimester) scans are generally considered the most accurate for dating.
- Trimesters: first trimester to 13 weeks 6 days, second from 14 weeks 0 days to 27 weeks 6 days, third from 28 weeks 0 days.
- Term definitions: early term 37+0 to 38+6, full term 39+0 to 40+6, late term 41+0 to 41+6 and post-term from 42+0, as defined by the American College of Obstetricians and Gynecologists (ACOG).
- Months of pregnancy: the common nine-month grouping of weeks; months are always approximate.
Fertility tools
- Ovulation: about 14 days (the typical luteal phase) before the next expected period; the fertile window is the five days before ovulation plus ovulation day, reflecting sperm survival of up to about five days and egg survival of about a day.
- Implantation: 6–12 days after ovulation, most often 8–10 days, based on studies that measured hCG daily from ovulation.
- Pregnancy test timing: reliable from the first day of a missed period, or about 21 days after unprotected sex if the period date is unknown — the guidance commonly given by national health services.
Pregnancy health tools
- Weight gain: 2009 Institute of Medicine (now National Academy of Medicine) recommendations by pre-pregnancy BMI, including provisional twin ranges.
- hCG doubling time: standard exponential-growth formula: hours × ln 2 ÷ ln(second ÷ first result).
- Contraction timer: frequency is measured start to start; the 5-1-1 pattern (about 5 minutes apart, about 1 minute long, for 1 hour) is a widely used guide.
- Pregnancy calories: Mifflin–St Jeor resting energy × activity factor, plus the Institute of Medicine’s extra energy: 0, 340 and 452 kcal a day by trimester and about 330 kcal for exclusive breastfeeding.
- Twin timing: ACOG guidance on delivery timing for uncomplicated twin pregnancies by chorionicity.
- Kick counter: follows current advice to learn your baby’s normal pattern and seek care promptly for reduced movements, with the traditional “time to 10 movements” method as an option.
Baby tools
- Baby age: calendar days, weeks and months from the date of birth.
- Growth percentiles: WHO Child Growth Standards (birth to 24 months) for weight-for-age, length-for-age and head circumference-for-age, using the LMS method with WHO’s adjustment beyond ±3 SD; data from the CDC’s published WHO growth chart files.
- Formula amounts: the common guide of about 150 ml per kg per day (2.5 oz per lb), usually not more than about 32 oz (950 ml) a day.
- Sleep: total sleep ranges from sleep medicine expert recommendations; wake windows and nap counts are common practical guides.
- Corrected age: chronological age minus the weeks born before 40 weeks, commonly used for babies born before 37 weeks until about age 2.
Dates and accuracy
All calculations count whole days on the Gregorian calendar, so month lengths and leap years are handled correctly, and they are checked by automated tests. We review our content regularly. If you think something is wrong, please contact us.
Our tools provide general information and estimates only — see our Medical Disclaimer.