Summary of this article

A detailed explanation of how to calculate the number of weeks of pregnancy. It also introduces how to calculate from the date of last menstrual period or ovulation, and the growth process of the fetus according to the number of weeks.

The calculation of the number of weeks of pregnancy is based on the start of the last menstrual period. The following steps explain this in more detail.

Check the start date of the last menstrual period: this is the starting date for the number of weeks of pregnancy.
Calculation of expected delivery date: The expected delivery date is 280 days (40 weeks) after the start of the last menstrual period. This is considered the standard gestational period.
Calculate the current number of weeks of pregnancy: the difference between today’s date and the start of the last menstrual period is converted into weeks.
Ultrasound correction: an ultrasound scan in early pregnancy may correct the number of weeks of gestation based on actual fetal growth. This is done if the last menstrual period start date is not accurate or if the date of ovulation differs from the normal date.

Example

If the last menstrual period started on 1 January 2023, the expected delivery date would be 8 October 2023 (280 days later), which would be 31 weeks as of 1 August 2023.

Points of Attention:

The number of weeks of pregnancy is very important to monitor the health of the mother and the fetus. For this reason, regular visits to the doctor are recommended.

Using this method, you can easily calculate your gestational week and expected delivery date, but the exact number of weeks is best confirmed by a doctor.

Pregnancy Weeks Calculator

Please enter the date of your last menstrual period：

The current number of weeks of pregnancy is:

医師監修監修日：2024年8月30日

岡博史（おかひろし）

医師・医学博士／ヒロクリニック統括院長

YouTubeチャンネルなどを通じて、遺伝子やNIPT（新型出生前診断）についてわかりやすく発信しています。

略歴

1996年慶應義塾大学医学部卒業
2004年慶應義塾大学医学博士号取得
2005年慶應義塾大学皮膚科学教室助手
2008年ヒロ皮フ形成クリニック開業
2009年医療法人社団福美会理事長
2015年医療法人社団福美会理事

資格・所属

CAPラボディレクター
日本皮膚科学会皮膚科専門医
日本医師会産業医
東京衛生検査所指導監督医

発信・関連リンク

この記事は、ヒロクリニックNIPPTの編集・監修体制にもとづき、資格を持つ医師が内容を確認しています。

医師監修監修日：2024年8月30日

堤修一（つつみしゅういち）

医師・医学博士／ヒロクリニック博多駅前院院長

略歴

1993年東京大学医学部医学科卒業
2016〜2019年東京大学先端科学技術研究センター准教授

発信・関連リンク

プロフィール詳細

この記事は、ヒロクリニックNIPPTの編集・監修体制にもとづき、資格を持つ医師が内容を確認しています。

参考文献

Committee on Obstetric Practice, American Institute of Ultrasound in Medicine, Society for Maternal-Fetal Medicine. Committee Opinion No. 700: Methods for Estimating the Due Date. Obstet Gynecol. 2017 May;129(5):e150-e154. doi: 10.1097/AOG.0000000000002046.
Robinson HP, Fleming JE. A critical evaluation of sonar "crown-rump length" measurements. Br J Obstet Gynaecol. 1975 Sep;82(9):702-710. doi: 10.1111/j.1471-0528.1975.tb00710.x.
Butt K, Lim K; Diagnostic Imaging Committee; SOGC Genetics Committee. Determination of gestational age by ultrasound. J Obstet Gynaecol Can. 2014 Feb;36(2):171-181. doi: 10.1016/S1701-2163(15)30664-8.
Mittendorf R, Williams MA, Berkey CS, Cotter PF. The length of uncomplicated human gestation. Obstet Gynecol. 1990 Jun;75(6):929-932.
Campbell S. Ultrasound measurement of the fetal biparietal diameter in early pregnancy. Clin Obstet Gynaecol. 1974 Dec;1(3):507-518.