Estimation of Gestational Age and Prediction of Expected Date of Delivery

Pregnancy, also known as gestation, is the period between conception and birth, during this time fetus grows and develops inside the mother’s womb (uterus). Accurate dating of pregnancy is important to improve outcomes.

Gestational Age is the common term used during pregnancy to describe how far along the pregnancy is. The duration of pregnancy can be calculated in terms of 10 lunar months or 9 months and 7 days or 280 days or 40 weeks, calculated from the first day of the last menstrual period.

Gestational age is the duration of pregnancy calculated from the first day of the last menstrual period (LMP).

Calculator to Calculate EDD (Expected Date of Delivery)

Babies born before 37 weeks of pregnancy are considered premature. Babies born after 42 weeks are considered post-mature and infants born between 38-40 weeks are considered term babies.

In 20-30 per cent of significant cases, the patient either fail to remember the LMP or report inaccurately. When the conception occurs during the lactation amenorrhoea period or soon following the withdrawal of contraceptive pills or in cases with bleeding in early pregnancy, the matter becomes complicated. The following parameters can be used in predicting gestational age with a good degree of accuracy.

Patient’s Statement:

The estimated due date or expected date of delivery (EDD) is the date that spontaneous onset of labour is expected to occur.

  1. Date of fruitful coitus– In certain cases where the patient forgets her last LMP date, she is asked to remember the date of single fruitful coitus, it is quite reliable to predict the expected date of delivery with accuracy of 50% within 7 days on either side. 266 days are to be added to the date of single fruitful coitus to calculate the expected date.
  2. Naegel’s formula– Women whose periods are regular and remember their LMP date, it is a very useful and commonly practiced method to calculate the expected date. It is estimated by adding 9 months and 7 days (280 days) to the first day of the last menstrual period (LMP). For example– Lets consider a women’s first day of last menstrual period is 2.1.2022, then add 9 months and 7 days to estimate the EDD.

LMP = 02 . 01. 2022 + 9 months and 7 days

EDD = 09. 10. 2022

Estimation of due or expected date of delivery (EDD)
Fig: Estimation of expected date of delivery(EDD)

3. Date of quickening– Quickening is the perception of active fetal movements by the mother about 18 weeks and 2 weeks earlier in multigravida. The delivery date can be estimated by adding 22 weeks in primigravidae and 24 weeks in multigravidae to the date of quickening.

Clinical records:

  1. Size of the uterus prior to 12 weeks more precisely corresponds with the period of amenorrhoea.
  2. Palpation of the fetal parts at the earliest by 20th week.
  3. Auscultation of fetal heart rate ( FHR ) at the earliest by 18 to 20 weeks using ordinary stethoscope or by using Doppler at 10th week.

Investigation records:

  1. Recording of positive pregnancy test using immunological principle at first missed period by earliest.
  2. Ultrasonographic findings at the earliest are gestational sac at 5 weeks and measurement of crown rump length (CRL) detected at 7 week.

Objective signs:

Some of the below-mentioned signs are useful in estimating the due date and gestational age.

  1. Height of the uterus
  2. Lightening (engagement of the presenting part)
  3. Size of the fetus, change in uterine shape, volume of liquor amnii, hardening of the skull and girth of the abdomen are of value in assessing the maturity of the fetus
  4. Vaginal examination: If the cervix becomes shorter and dilated the labour is fairly not far off but in some cases labour may start even with long and closed cervix.


  1. Sonography– Ultrasound measurement of the fetus in first trimester is the most accurate method to confirm gestational age. Through ultrasound CRL (Crown Rump Length), BPD (Bi-parietal Diameter), HC (Head Circumference), AC (Abdominal Circumference) and FL (Femur Length) can be measured.

Gestational age determined by sonographic measurement in the first or second trimester should be compared with the menstrual age. In clinical practice, when the difference between the two is less than 10 days, the EDD derived from the LMP is confirmed. When the difference is more, the EDD should be based on ultrasonographic fetal biometry.

2. X-ray- Done in rare cases, when there is the presence of complications in pregnancy.


In the above post, we have discussed gestational age, calculation of the expected date of delivery, and various parameters used in predicting gestational age.

Thank you all, hope you find this useful.

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