Why can the due date between pregnancy ultrasounds change?

This is an automatically translated article.

The article was professionally consulted by Dr. Nguyen Anh Tu - Doctor of Obstetric Ultrasound - Prenatal Diagnosis - Obstetrics Department - Vinmec Hai Phong International General Hospital.
A normal pregnancy lasts an average of 280 days (about 40 weeks) from the first day of your last menstrual period before becoming pregnant. However, changing the due date is also common, some cases of ultrasound 2 weeks deviation from the actual date of birth can also occur. Very few women actually give birth on their due date.

1. How can the due date be calculated?

A woman with a regular 28-day menstrual cycle can estimate her due date in two ways:
Naegele's rule Naegele's rule has a simple formula as follows: Add 7 to the first day of the last menstrual period before pregnancy then subtract 3 months. For example, if the first day of your last period is November 1, 2017, your due date is August 8, 2018.
Calculation cycle Another way you can calculate your due date. I am using gestational age rotation. This method is also commonly used by clinicians. The first step is to mark the first day of your last period on the pivot table. At that time, pregnant women only need to read the due date at the position of the rotating needle. It should be noted that this is only a way to estimate the time that the baby will be born, the probability of being born exactly on the due date is not too high.
Both options above cannot be done if the pregnant woman does not remember her last menstrual period. This is also common, but there are other ways to know your due date when you don't remember your period such as:
Doctors can estimate your due date if you know your last period occurred in a particular week body. Your doctor will use a pregnancy ultrasound to calculate or change your due date. For women with menstrual cycles longer than 28 days, the gestational age wheel can still be used, but with some adjustments. The second half of the menstrual cycle lasts about 14 days. This is the time when you ovulate to prepare for the next cycle. If your cycle lasts 35 days, ovulation usually falls around day 21 of your cycle. Once the date of ovulation is known, the pregnant woman can adjust the first day of the last menstrual period to determine the due date on the gestational age wheel.

For example, if the menstrual cycle is 35 days long and the first day of the last period is November 1st:
Add this 21 to get the date of November 22 Then subtract 12 to find the adjustment for the first day of the period New last period, we have 8 November. After adjusting for the first day of last period, mark it on the gestational age wheel and determine the due date in the same way.

2. Fetal ultrasound determines the due date

When the doctor orders a pregnancy ultrasound to know the due date, the doctor will note the two expected birth dates, including the results based on the last menstrual cycle and based on the indicators on the ultrasound. Rarely do these two due dates coincide. Doctors usually won't change your due date based on your last period if it's regular 28-day cycles, unless there's a big deviation from your due date with a fetal ultrasound. Expected date of delivery is determined by fetal ultrasound based on fetal biometric indicators such as head buttock length, head circumference, femur length, ..
If fetal ultrasound is performed many times throughout the pregnancy Periodically, each ultrasound result will have a different due date based on the most recent readings. However, the due date should not be changed if performed during a 13-week pregnancy ultrasound or during a second- and third-trimester pregnancy. The due date is more accurate if taken early in pregnancy. Fetal ultrasound in the later days of the menstrual cycle is often used to assess fetal physical development rather than predicting due date.

3. Why do doctors change the due date in some cases?

Your doctor will usually decide to change your due date if your baby is much smaller or larger than the average size for that gestational age. Usually, the doctor will order a pregnancy ultrasound to determine the due date when a woman can't remember her last period or has an irregular menstrual cycle or when she's pregnant while using birth control pills.
Fetal ultrasound allows the doctor to measure the length of the top of the buttocks (in English is crown-rump length, CRL), the length connecting the two points of the fetus. In the first trimester of pregnancy, measuring the length of the top of the buttocks gives the most accurate results of the due date. Your doctor may change your due date based on your ultrasound measurements. This usually happens if the difference between the due date on ultrasound and the due date calculated according to the last menstrual cycle is more than 1 week.
In the 2nd trimester of pregnancy, fetal ultrasound is less accurate and the doctor will not adjust the due date if it is more than 2 weeks away.
The third trimester of pregnancy is the least accurate time to determine the due date. Based on ultrasound results, the error can be up to 3 weeks, so doctors rarely change the due date in the last 3 months of pregnancy. Indications for repeat fetal ultrasound during pregnancy are often used to obtain additional information to assess fetal development. As such, the due date between pregnancy ultrasounds can vary, but estimating gestational age in the first weeks of pregnancy is often the most accurate. During this time, the fetal growth rate is quite stable. Later, the fetal growth rate is not constant, changing much as the pregnancy develops. Therefore, fetal ultrasound in later stages should not be used to predict gestational age because it is not very accurate. Fetal ultrasound is not recognized as part of antenatal care and should therefore only be performed for medical purposes.

At Vinmec International General Hospital, there is a package maternity service as a solution to help pregnant women feel secure because of the companionship of the medical team throughout the pregnancy. When choosing Maternity Package, pregnant women can:
The pregnancy process is monitored by a team of qualified doctors Regular check-up, early detection of abnormalities Maternity package helps to facilitate the process. birthing process Newborns get comprehensive care

Please dial HOTLINE for more information or register for an appointment HERE. Download MyVinmec app to make appointments faster and to manage your bookings easily.

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

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