Why can the due date between pregnancy ultrasounds change?

The article was consulted professionally by Dr. Nguyen Anh Tu – Doctor of Obstetrics ultrasound – Pre-birth diagnosis – Obstetrics department – Share99 Hai Phong International Health Hub.

A normal pregnancy lasts an average of 280 days (about 40 weeks) from the first day of the last menstrual cycle before pregnancy. However, a change in the date of birth also usually occurs, some cases of pregnancy ultrasound deviate by 2 weeks from the actual date of birth. Very few women actually give birth on the day of birth.

1. How can the date of birth be calculated?

A woman with an even, 28-day menstrual cycle can estimate her own date of birth in two ways:

  • Naegele Rules

The Naegele rule has the following simple calculation formula: Add 7 on the first day of the last menstrual cycle before pregnancy then subtract 3 months. For example, if the first day of the last menstrual cycle is November 1, 2017, the date of birth is August 8, 2018.

  • Pregnancy age calculation rotation

Another way that a pregnant woman can calculate her date of birth is to use a rotation to calculate the gestational age. This method is also often used by clinicians. The first step should mark the first day of the last menstrual cycle on the rotating table. Then the pregnant woman only needs to read the date of birth at the location of the spinning needle head. It should be noted that this is only a way to estimate the period of time that the baby will be born, the exact probability of giving birth on the day of birth is not too high.

Neither of these options can be done if the pregnant woman does not remember her last menstrual cycle. This is also a common thing, but there are still other ways to know the date of birth when not remembering the menstrual cycle such as:

  • Your doctor may estimate your date of birth if you know the last menstrual cycle appears in a particular week.
  • The doctor will use a pregnancy ultrasound to calculate or change the date of birth.

For women with menstrual cycles longer than 28 days, the gestational age rotation can still be used however a few adjustments are required. The second half of the menstrual cycle lasts about 14 days. This is the moment when ovulation prepares for the next cycle. If the menstrual cycle lasts 35 days, the day of ovulation usually falls around the 21st day of the cycle. Once the date of ovulation is known, the pregnant woman can adjust the first day of the last menstrual cycle to determine the date of birth on the rotation of gestational age.

Pregnancy age calculation rotation

Pregnancy age calculation rotation

For example, if the menstrual cycle lasts 35 days and the first day of the last menstrual cycle is November 1:

  • Plus 21, we have November 22nd.
  • Then subtract for 12 to find adjustments to the first day of the new menstrual cycle, we have November 8th.
  • After having adjusted the first day of the last menstrual cycle, mark it on the rotation that calculates the gestational age and determines the date of birth by analogy.

2. Pregnancy ultrasound determines the date of birth

When the doctor has an ultrasound of the pregnancy to know the date of birth, the doctor will note the two expected birth days, including the results based on the last menstrual cycle and based on the indicators on the ultrasound. Rarely do these two birth days coincide. The doctor usually does not change the date of birth based on the last menstrual cycle if it is that the menstrual cycles all last 28 days, unless there is a much deviation from the date of birth by pregnancy ultrasound. The date of birth is determined by an ultrasound of the pregnancy based on the biometric indicators of the pregnancy such as the length of the buttocks head, the circumferentimeter of the head, the length of the femur,..

If a pregnancy ultrasound is performed multiple times throughout the pregnancy, each ultrasound result will have a different date of birth based on the most recent indicators. However, the date of birth should not be changed if performed during an ultrasound of pregnancy 13 weeks 1 or later or during pregnancy ultrasound in the 2nd and 3rd quarters of pregnancy. The date of birth is more accurate if performed in the early stages of pregnancy. Pregnancy ultrasound in the latter days of the menstrual cycle is often used to assess the physical development of the pregnancy rather than to estimate the date of birth.

Does pregnancy ultrasound affect anything

Pregnancy ultrasound helps determine the date of birth

3. Why does the doctor change the date of birth in some cases?

The doctor will usually decide to change the date of birth if the fetus is much smaller or larger than the average fetal size corresponding to that gestational age. Usually, the doctor will prescribe a pregnancy ultrasound that determines the date of birth when the pregnant woman does not remember the last menstruation or the uneven menstrual cycle or during pregnancy while using the contraceptive pill.

Pregnancy ultrasound allows the doctor to measure the length of the buttocks (crown-rump length, CRL), the length of which connects the two points of the pregnancy. In the first quarter of pregnancy, the measurement of the length of the buttocks gives the most accurate date of birth. The doctor may change the date of birth based on the measurement of the dimensions on the ultrasound. This usually occurs if the deviation between the expected date of birth on the ultrasound and the expected date of birth in the last menstrual cycle is greater than 1 week.

In the second quarter of pregnancy, the pregnancy ultrasound is less accurate and the doctor will not adjust the date of birth if the deviation is more than 2 weeks.

The third quarter of pregnancy is the least accurate time to determine the date of birth. If the ultrasound results are based, the error can be up to 3 weeks so the doctor rarely changes the due date in the last 3 months of pregnancy. Repeated fetal ultrasound inations throughout pregnancy are often used for additional information to assess fetal development. As such, the date of birth between pregnancy ultrasounds may vary, however the estimation of gestational age in the first weeks of pregnancy is usually most accurate. During this period, the rate of development of the fetus is quite stable. Later, the growth rate of the fetus is not constant, changing many times when the pregnancy develops. Therefore, pregnancy ultrasound in later stages should not be used to predict gestational age because it is incorrect. Pregnancy ultrasound is not recognized as part of pre-maternity care, as it should only be done for medical purposes.

Childbirth, labor

The date of birth will change after each ultrasound

At Share99 International Health Hub, there is a full maternity service as a solution to help pregnant mothers feel secure because there is a team of doctors throughout the pregnancy. When choosing a full-service maternity, a pregnant woman can:

  • The process of pregnancy is monitored by a team of specialists
  • Regular examination, early detection of abnormalities
  • Package maternity for the convenience of childbirth
  • Infants receive comprehensive care

For direct advice, please click hotline number or register online HERE. In addition, you can register for remote consultation HERE

Source: healthline.com, babycenter.com, acog.org/Patients

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  • How is the date of birth calculated? Is it dangerous to get pregnant beyond the date of birth?
  • 40-week pregnancy monitoring with no signs of birth

About: Minh Quynh

b1ffdb54307529964874ff53a5c5de33?s=90&d=identicon&r=gI am the author of Share99.net. I had been working in Vinmec International General Hospital for over 10 years. I dedicate my passion on every post in this site.

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