Determining a pregnancy’s timeline: The role of ultrasound in estimating gestational age

Determining a pregnancy’s timeline: The role of ultrasound in estimating gestational age

by | Oct 13, 2023 | Pregnancy

Determining the due date of a pregnancy is an important part of prenatal care. With the help of ultrasound technology, it is possible to pinpoint a baby’s gestational age, due date, and other important clinical information with a high degree of accuracy. Using this diagnostic information, doctors and obstetric specialists can provide the most appropriate level of care. 

If you’ve been booked in for a dating scan at QUFW, or you are interested in learning more about prenatal diagnostics and the process and technology behind pregnancy dating, this fact sheet may address some of your questions and concerns.

This fact sheet answers these questions:

What does it mean to “date” a pregnancy, and why is a “dating scan” important?

To “date” a pregnancy means to determine how far along the pregnancy is and, subsequently, estimate the date the baby will be born (the due date). 

This is typically calculated based on the baby’s gestational age, which starts from the first day of the mother’s last menstrual period (LMP) and includes the two weeks prior to conception. The due date, often referred to as the estimated date of delivery (EDD), is approximately 40 weeks from the LMP.

The EDD from the LMP is calculated under the assumption of a regular 28 day menstrual cycle. In the event of irregular menstrual cycles or a shorter/longer cycle, EDD may differ from what is calculated by general dating apps or calculators.

According to the Australasian Society of Ultrasound in Medicine (ASUM) Guideline for the performance of First Trimester Ultrasound, the EDD by LMP (adjusted for cycle length) should be used unless the LMP is unknown or if the gestational age by the CRL differs significantly or by more than 5 days. EDD by assisted reproduction dates (for example, IVF) should only be adjusted with extreme caution.

Guidelines-for-the-Performance-of-First-Trimester-Ultrasound.pdf (asum.com.au)

Dating a pregnancy is important for several reasons:

  • Prenatal care planning: Accurately dating a pregnancy allows doctors and obstetric specialists to schedule necessary prenatal tests, screenings, and ultrasounds at appropriate times. Different tests are conducted at specific times throughout the pregnancy, so knowing the gestational age ensures timely and appropriate care.
  • Monitoring fetal growth: An accurate gestational age provides a benchmark for evaluating the fetus’ growth and development. If the fetus is larger or smaller than expected for its gestational age, it may indicate potential health issues or complications.
  • Anticipating birth: Knowing the EDD helps parents-to-be prepare for the baby’s arrival, both emotionally and logistically. It aids in planning maternity leave, organising baby-related purchases, and mentally preparing for labour and childbirth.
  • Medical interventions: If complications arise, or if the baby doesn’t arrive by the expected due date, medical decisions such as inducing labour or scheduling a C-section can be informed by the accurate dating of the pregnancy.
  • Assessing potential complications: In cases where pregnancies go beyond 42 weeks (post-term), there might be increased risks to both the mother and baby. Conversely, preterm births (before 37 weeks) can also present challenges. Knowing the accurate gestational age helps doctors and obstetric specialists make informed decisions regarding the management of these situations.

 

What is a dating scan?

A dating scan, often known as a first trimester ultrasound, is an ultrasound examination best carried out between 7 and 12 weeks of pregnancy. Its primary purpose is to determine the gestational age of the pregnancy and, consequently, estimate a due date. Here are some of the key objectives of a dating scan:

  • Determination of gestational age: By measuring the baby from the top of its head (crown) to the bottom of its spine (rump), known as the crown-rump length (CRL), doctors and obstetric specialists can determine the age of the pregnancy. This measurement is particularly reliable during the early stages of pregnancy when developmental differences between individual fetuses are minimal. It is most reliable between 7-10 weeks. 
  • Estimation of due date: Once the gestational age has been determined, a due date can be estimated. Knowing the expected due date helps in planning subsequent prenatal care, preparing for childbirth, and identifying if there are any deviations from the expected fetal growth or development in later stages.
  • Additional observations: While the primary objective of a dating scan is to determine gestational age, the scan can also provide other important information. For example, it can confirm whether the pregnancy is intrauterine (developing within the uterus), ascertain if there are multiple pregnancies (e.g., twins or triplets), and detect the fetal heartbeat.

 

How accurate is an ultrasound scan in determining gestational age?

Ultrasound is a reliable tool for determining gestational age, especially when conducted in the first trimester of pregnancy. However, its accuracy can vary based on the timing of the ultrasound and certain other factors. Like any measurement, there may be measurement error that needs to be considered. A vaginal ultrasound may be necessary to have more accuracy in visualising and measuring the length of your baby.

First Trimester Ultrasounds (7-13 weeks): 

These are considered the most accurate timing for dating a pregnancy. During this period, the fetus undergoes rapid developmental changes, and variations in growth among different fetuses are minimal. The crown-rump length (CRL), which is the measurement from the top of the baby’s head (crown) to the bottom of its spine (rump), is used to determine gestational age. The most accurate timing is ideally between 7-11 weeks gestation.

Second Trimester Ultrasounds (14-24 weeks): 

While still relatively accurate, the margin of error increases as the pregnancy progresses. In the second trimester, gestational age is often estimated by measuring various fetal body parts like the head circumference, abdominal circumference, and femur length. 

Third Trimester Ultrasounds (after 24 weeks): 

By this stage, individual differences in fetal growth become more pronounced, making ultrasounds less precise for dating purposes.

 

What are the various ultrasound markers and parameters used for age determination?

A ultrasound scan uses the following markers and parameters to determine the age, and other factors, of a pregnancy:

  • Crown-Rump Length (CRL): Used primarily in the first trimester, CRL measures the length of the fetus from the top of its head (crown) to the bottom of its spine (rump). This measurement is the most accurate in the early stages, specifically between 7 and 11 weeks of gestation.
  • Biparietal Diameter (BPD): Measured during the second and third trimesters, BPD is the diameter of the fetus’s head, taken from one parietal bone to the other.
  • Head Circumference (HC): Also taken during the second and third trimesters, this measurement encompasses the entire circumference of the fetal head. Along with the BPD, the HC is taken in the second and third trimesters.
  • Abdominal Circumference (AC): This measurement, which represents the circumference of the fetal abdomen, gives insights into the baby’s liver size and overall growth. It’s usually taken in the second and third trimesters.
  • Femur Length (FL): Representing the length of the fetal thigh bone, FL is a standard measurement for assessing the length of the bones and overall skeletal development.
  • Humerus Length (HL): The length of the fetus’s upper arm bone can also be an indicator of skeletal development.

 

Can multiple pregnancies (like twins or triplets) affect the accuracy of gestational age determination via ultrasound?

Yes, multiple pregnancies, such as twins or triplets, can present challenges when determining gestational age via ultrasound, but it’s not necessarily that the measurements themselves are less accurate. Rather, the complications and variables introduced by the presence of more than one fetus can affect the interpretation and the application of these measurements. Here’s how:

  • Individual growth rates: Especially as the pregnancy progresses, twins or triplets might grow at different rates. This is especially common in monochorionic pregnancies (where the fetuses share a placenta). One fetus may receive more nutrients than the other, leading to size discrepancies. If there’s a significant size difference, determining an average gestational age might be more challenging.
  • Positioning: With multiple fetuses in the uterus, getting clear ultrasound images can be more challenging due to the positions of the fetuses. Accurate measurements require clear images, and if one or more fetuses are not in an ideal position, it can be difficult to obtain precise measurements.
  • Type of twin pregnancy: The type of twin pregnancy can also influence growth. Monochorionic twins (those sharing a placenta) can sometimes face conditions like Twin-to-Twin Transfusion Syndrome (TTTS), where blood supply is unevenly distributed between the twins. This can affect the growth of one or both babies and might complicate gestational age determination based on size.
  • General increased variability: Multiple pregnancies naturally have a broader range of what’s considered “normal” in terms of growth and development. This increased variability can make it harder to pinpoint gestational age based solely on measurements, especially in the second and third trimesters.

 

What should expectant mothers know before going in for an ultrasound meant for dating purposes?

When expectant mothers are preparing for an ultrasound meant for dating purposes, there are several key pieces of information and tips they should be aware of to ensure a smooth and informative experience:

  • Purpose of the ultrasound: It’s essential to understand that the primary goal of a dating ultrasound is to determine the gestational age of the pregnancy and estimate the due date. This can be especially helpful if there’s uncertainty about the date of the last menstrual period.
  • Full bladder may be needed: For early pregnancy ultrasounds, a full bladder can help push the uterus up out of the pelvis, but can also act as a ‘window’ to see the uterus behind the bladder, offering clearer images of the fetus. 
  • Transvaginal ultrasound: Depending on how early the pregnancy is, an abdominal ultrasound might not provide clear images. In such cases, a transvaginal ultrasound, where an ultrasound transducer is gently inserted into the vagina, might be recommended. It provides more detailed images, especially during the very early stages of pregnancy. A transvaginal ultrasound is performed with an empty bladder.
  • It’s diagnostic, not just pictorial: While many expectant parents eagerly await that first “picture” of their baby, it’s crucial to remember that the primary purpose is diagnostic. The images might not always look like the clear baby profiles seen in later-term ultrasounds.
  • Multiple pregnancies: The ultrasound can also detect if there are multiple pregnancies, like twins or triplets. If this is the case, subsequent monitoring and care might be adjusted accordingly.
  • Limitations: Ultrasound, especially when done early in the pregnancy, is quite accurate for dating purposes, but it does have a margin of error. 
  • Safety: Ultrasound uses sound waves and not radiation, making it safe for both the mother and the baby. There are no known harmful effects when used appropriately.
  • Results and interpretation: Occasionally, during an ultrasound if there is a discrepancy with dating it may require you to be rescanned in 7-10 days to confirm the growth of the fetus and presence of a heart beat. At QUFW, one of our specialist obstetrician and gynaecologists will review and interpret the results and share with yourself and your referring doctor. Your referring doctor will decide the EDD for you to use.
  • Emotional Preparedness: While many ultrasounds go smoothly, there’s always a possibility of unexpected findings, such as detecting potential complications or issues. It’s essential to be emotionally prepared and remember that the healthcare team is there to provide support and guidance.

A dating ultrasound is a significant step in the prenatal journey. It offers important insights into a pregnancy’s progress, and sets the stage for subsequent care. 

If you have any other questions about your dating scan, or ultrasound, please speak to your doctor or obstetric specialist. 

Further reading

Guidelines-for-the-Performance-of-First-Trimester-Ultrasound.pdf (asum.com.au)

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