[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Nursing](https://durgical.com/en/nursing/) › [Maternal and child nursing](https://durgical.com/en/nursing/areas/maternal-child/) › Obstetric calculations: gestational age, estimated date of birth, gravidity and parity # Obstetric calculations: gestational age, estimated date of birth, gravidity and parity - Calculation Gestational age and the estimated date of birth come from the first day of the last period; the obstetric history is counted the way the perinatal record asks. Updated Nov 13, 2026 ## Key points - Educational example, not a clinical order. - A reliable LMP (regular cycles, no recent hormonal contraception) is the gold standard for dating. - Gestational age is recorded in completed weeks and days. - Naegele and the 280-day count can differ by a day or two, because months have different lengths. - «Previous pregnancies» (gestas previas) does not include the current pregnancy. ## What it is Gestational age is counted in weeks from the first day of the last menstrual period (LMP) to today. The estimated date of birth is the day of week 40. It can be read on the CLAP gestogram, counted as 280 days on a calendar, or worked out with a rule such as Naegele's. When the LMP is unknown or unreliable, an early ultrasound gives the age. The perinatal record counts the previous pregnancies and their outcomes in fixed fields, which the nurse fills from the woman's own account. ## Formulas - Gestational age = days from the first day of the LMP to today ÷ 7; the remainder is the extra days. - Naegele: first day of the LMP + 7 days, − 3 months (add one year when the result falls in the next year). - 280-day rule: count 280 days on a calendar from the first day of the LMP. - Wahl: first day of the LMP + 10 days, − 3 months. Pinard: last day of the LMP + 10 days, − 3 months. - Abortion in the perinatal record: expulsion before 22 weeks or under 500 grams, spontaneous or induced. Preterm birth: from 22 to before 37 weeks. - Early ultrasound: crown–rump length, measured between weeks 8 and 13, is the most faithful measure (error about ± 7 days). ## How to calculate it - Ask calmly, once, for the first day of the last period and write it as day-month-year — the whole calculation rests on it. - Check that cycles are regular and there was no recent hormonal contraception — otherwise the date is not reliable. - Count the days from the LMP to today and divide by 7 — the result is completed weeks plus days. - Work out the estimated date with the gestogram, 280 days or Naegele — use one method and record which. - If the LMP is unknown or doubtful, use the early ultrasound and mark in the record what the age is based on — avoids two ages in one record. - Ask the outcome of each previous pregnancy: births (vaginal or caesarean), abortions, ectopics, live and stillborn, children alive — fills the record. - Check the totals with the woman before closing the record — memory errors are common. ## Worked examples with made-up data | Data | Calculation | Result | | --- | --- | --- | | LMP 10/03/2026 (Naegele) | 10 + 7 = 17; March − 3 months = December | Estimated date 17/12/2026 | | LMP 10/03/2026 (280-day rule) | 10/03/2026 + 280 days on the calendar | Estimated date 15/12/2026, two days before Naegele | | LMP 28/01/2026 (Naegele) | 28 + 7 = 4 February; February − 3 months = November; + 1 year | Estimated date 04/11/2026 | | LMP 03/08/2026, visit on 13/11/2026 | 102 days ÷ 7 = 14 weeks, remainder 4 | Gestational age 14 weeks and 4 days | | Pregnant now; before: one vaginal birth, one caesarean, one miscarriage at 8 weeks; two children alive | The current pregnancy is not counted | Previous pregnancies 3; births 2 (vaginal 1, caesarean 1); abortions 1; alive 2 | ## Common mistakes - Counting the current pregnancy in «previous pregnancies». - Forgetting to add the year with Naegele when the LMP is in January, February or March. - Using the last day of the period with Naegele (Pinard uses the last day; Naegele uses the first). - Rounding up the weeks instead of recording completed weeks. ## Double-checking - Check the date with a second method (gestogram or calendar) and compare it with the early ultrasound when there is one. - Read the obstetric history back to the woman and correct it with her. - Mark in the record whether the age is reliable by LMP, by ultrasound, or both. ## Test yourself Think of the answer, then tap the question to see it. ## What does Naegele's rule do with the first day of the last period? It adds 7 days and subtracts 3 months, adding one year when needed. ## A woman's LMP was 01/04/2026. How many weeks pregnant is she on 01/07/2026 (91 days)? 13 weeks exactly (91 ÷ 7 = 13). Educational example, not a clinical order. ## Is the current pregnancy counted in «previous pregnancies» on the perinatal record? No. It records only earlier pregnancies; a first pregnancy is written as 00. ## Which ultrasound measure dates a pregnancy best, and when? Crown–rump length, between weeks 8 and 13; its error is about plus or minus 7 days. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/nursing/obstetric-calculations-gestational-age-estimated-date-of-birth-gravidity-and-par/) ## Elsewhere in DurgiPedia - [The placenta and pregnancy hormones](https://durgical.com/en/physiology/the-placenta-and-pregnancy-hormones/) ## Connections ## Related 2 - [Prenatal care: the nursing visit and warning signs](https://durgical.com/en/nursing/prenatal-care-the-nursing-visit-and-warning-signs/) - [The prenatal check: fundal height, Leopold manoeuvres, fetal heart, fetal movements and antenatal fetal tests](https://durgical.com/en/nursing/the-prenatal-check-fundal-height-leopold-manoeuvres-fetal-heart-fetal-movements/) [← All of nursing](https://durgical.com/en/nursing/) Educational content for students; it does not replace your institution's protocols, supervised clinical training or a prescriber's order. Dose-calculation examples use made-up numbers: they are educational, not a clinical order. Sources: [Salud sexual y reproductiva: guías para el continuo de atención de la mujer y el recién nacido focalizadas en APS, 3.ª ed. (CLAP/SMR 1577)](https://perinatal.bvsalud.org/wp-content/uploads/2023/08/CLAP1577.pdf) (Organización Panamericana de la Salud, CLAP/SMR, 2011); [Sistema Informático Perinatal (SIP): historia clínica perinatal, instrucciones de llenado y definición de términos, 2.ª ed.](https://www.paho.org/sites/default/files/1._sip_nmiss_instrucciones_de_llenado_0.pdf) (Organización Panamericana de la Salud, CLAP/SMR, 2017); [Gestational age](https://medlineplus.gov/ency/article/002367.htm) (MedlinePlus, U.S. National Library of Medicine, 2025). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/nursing/obstetric-calculations-gestational-age-estimated-date-of-birth-gravidity-and-par/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/nursing/obstetric-calculations-gestational-age-estimated-date-of-birth-gravidity-and-par/