Naegele's rule and the other methods behind our calculators
Every calculator on BabyMakers does three things a single-date calculator usually does not. It names the formula it used, it names where that formula comes from, and it shows the range around the answer instead of one falsely precise date.
Where the methods come from
- Due dates — Naegele's rule, 280 days from the first day of your last period (StatPearls, Naegele's rule), adjusted for your cycle length. When an ultrasound should change that date, we follow the American College of Obstetricians and Gynecologists' Committee Opinion 700 (2017, reaffirmed 2022): a change when a first-trimester scan differs from the last-period date by more than 7 days, or by more than 5 days before 9 weeks.
- Ovulation and the fertile window — the six days that end on the day of ovulation, made up of the five days before it and the day itself (Wilcox, Weinberg and Baird, New England Journal of Medicine, 1995; ACOG), with a luteal phase that averages 14 days and normally runs from 11 to 17 (American Society for Reproductive Medicine, 2021).
- Weight gain — the Institute of Medicine's 2009 ranges, now the National Academy of Medicine, shown as US guidance (IOM, 2009; ACOG Committee Opinion 548, 2013). The UK's National Institute for Health and Care Excellence does not recommend routine repeat weighing and sets no targets, and we say so beside them (NICE NG201, 2021; NICE NG247, 2025).
Ranges, not false precision
About 5 in 100 babies arrive on their estimated due date, and about 66 in 100 within a week of it (Khambalia and colleagues, 2013, in 18,708 single births after spontaneous labour in one Australian health district). So a due date here always comes with its spread, and fertile-window estimates carry one too: the fertile window can be highly unpredictable even when cycles are usually regular (Wilcox, Dunson and Baird, BMJ, 2000). Where we give a probability, the natural frequency comes first — "5 in 100" before "5%".
UK and US, flagged rather than glossed over
Pick your flag at the top of any page. Units, date formats and the guidance we show change with it, and where the two countries' guidance disagrees, we show both and say which is which.
What we store
You do not need an account, and we do not save what you enter into a calculator. Your entries are carried in the page address so that a result can be bookmarked, and like any address it can appear in our host's standard access logs. We use two small preference cookies in your own browser — the flag you pick and, if you tell us, which nation you are in, so the right emergency numbers show — plus the standard cookie that keeps the site's forms secure.
Information, not medical advice
Everything here is information and signposting. It is not a diagnosis, and it is not a substitute for your midwife, doctor or care team.
Sources
- American College of Obstetricians and Gynecologists, American Institute of Ultrasound in Medicine and Society for Maternal-Fetal Medicine. Methods for estimating the due date (Committee Opinion No. 700). Obstetrics & Gynecology. 2017;129(5):e150–e154. doi:10.1097/AOG.0000000000002046. Reaffirmed 2022.
- Naegele’s rule. StatPearls. StatPearls Publishing. NCBI Bookshelf NBK536986. Accessed 28 August 2026.
- Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation: effects on the probability of conception, survival of the pregnancy, and sex of the baby. New England Journal of Medicine. 1995;333(23):1517–1521. doi:10.1056/NEJM199512073332301. PMID 7477165.
- Wilcox AJ, Dunson D, Baird DD. The timing of the “fertile window” in the menstrual cycle: day specific estimates from a prospective study. BMJ. 2000;321(7271):1259–1262. doi:10.1136/bmj.321.7271.1259. PMID 11082086.
- American College of Obstetricians and Gynecologists. Trying to get pregnant? Here’s when to have sex. Last updated August 2023, last reviewed August 2025.
- Practice Committees of the American Society for Reproductive Medicine and the Society for Reproductive Endocrinology and Infertility. Diagnosis and treatment of luteal phase deficiency: a committee opinion. Fertility and Sterility. 2021;115(6):1416–1423. doi:10.1016/j.fertnstert.2021.02.010.
- Institute of Medicine and National Research Council. Weight Gain During Pregnancy: Reexamining the Guidelines. Washington, DC: National Academies Press; 2009. doi:10.17226/12584.
- American College of Obstetricians and Gynecologists. Weight gain during pregnancy (Committee Opinion No. 548). Obstetrics & Gynecology. 2013;121(1):210–212. doi:10.1097/01.AOG.0000425668.87506.4c.
- National Institute for Health and Care Excellence. Antenatal care (NG201). Published 19 August 2021; last reviewed 27 December 2024.
- National Institute for Health and Care Excellence. Maternal and child nutrition: nutrition and weight management in pregnancy, and nutrition in children up to 5 years (NG247). 2025.
- Khambalia AZ, Roberts CL, Nguyen M, Algert CS, Nicholl MC, Morris J. Predicting date of birth and examining the best time to date a pregnancy. International Journal of Gynecology & Obstetrics. 2013;123(2):105–109. doi:10.1016/j.ijgo.2013.05.007. PMID 23932061.