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CRL Calculator

Find our your Gestational Age (GA) which is an approximation of your current pregnancy age from the size of your baby. CRL or the Crown Rump Length can often be found in your first pregnancy ultrasound scan report.

London Pregnancy Clinic - Pregnancy Due Date Calculator for early pregnancy scans.

Enter your CRL to find out GA


What is CRL? How accurate is it for dating the pregnancy?

Crown-Rump Length (CRL) is a vital measurement obtained via ultrasound that assesses the length of the fetus from the top of its head (crown) to the bottom of its buttocks (rump). This measurement is typically taken during the first trimester of pregnancy, usually between the 6th and 13th week. CRL is instrumental in determining the gestational age of the fetus with significant accuracy, which, in turn, helps in calculating the expected date of delivery (EDD).

CRL is renowned for its precision in dating a pregnancy during the first trimester, often considered the gold standard for establishing gestational age. The accuracy of CRL in determining the gestational age is critical, as it can influence decisions regarding prenatal care and the management of the pregnancy. Studies have shown that CRL can predict the gestational age with an error margin of approximately 3 to 5 days, making it highly reliable. This level of precision is unmatched by other first-trimester measurements and becomes particularly useful for women who are uncertain about their last menstrual period or have irregular menstrual cycles.

The accuracy of CRL diminishes as pregnancy progresses, making early ultrasound examinations the optimal time for CRL-based dating. Establishing an accurate gestational age in the first trimester through CRL measurements is essential for several reasons:

  • It provides a baseline for monitoring fetal growth and development.
  • It aids in the assessment of the risk for chromosomal abnormalities.
  • It assists in the planning and timing of specific prenatal tests.
  • It helps in determining the appropriateness of interventions if the pregnancy is post-term or pre-term.

Frequently Asked Questions

You have a question? We have an answer.

Is my EDD accurate from CRL accurate?

he accuracy of your Estimated Due Date (EDD) derived from Crown-Rump Length (CRL) measurements during the first trimester is generally considered to be quite high. This method of dating a pregnancy is widely regarded as the most reliable in early pregnancy stages, primarily because fetal development follows a relatively standard pattern during this period, making CRL a consistent marker for gestational age.

CRL-based EDD calculations are particularly useful if you’re unsure about the date of your last menstrual period (LMP) or if you have irregular menstrual cycles. By measuring the length of the fetus from the top of its head to the bottom of its buttocks, healthcare professionals can accurately determine the gestational age and, consequently, predict the due date with a narrow margin of error.

Research indicates that EDD predictions based on first-trimester ultrasound scans, which include CRL measurements, can be accurate within 3 to 5 days. This level of precision is considered superior to due date estimations based on LMP or second-trimester ultrasound scans, which have wider margins of error.

However, it’s essential to note that no predictive method for determining the EDD is 100% accurate. Various factors, including the skill of the ultrasound technician, the quality of the ultrasound equipment, and the fetus’ position at the time of the scan, can influence the accuracy of CRL measurements. Despite these variables, first-trimester CRL assessments remain the gold standard for dating pregnancies.

What are the methods used to date a pregnancy?

Dating a pregnancy accurately is crucial for monitoring fetal development, scheduling prenatal tests, and planning for the birth. Various methods are employed to estimate the gestational age and determine the Expected Due Date (EDD). The accuracy of these methods can significantly impact prenatal care and decisions throughout the pregnancy. Here are the primary methods used:

1. Last Menstrual Period (LMP)

The LMP method calculates the gestational age by counting the days from the first day of the last menstrual cycle. This method assumes a 28-day cycle and ovulation occurring approximately 14 days from the start of the cycle. While widely used, its accuracy depends on the regularity of the menstrual cycle and the woman’s ability to recall the correct date.

2. Ultrasound Scanning

Ultrasound scanning is a highly reliable method for dating a pregnancy, especially when conducted in the first trimester.

  • Crown-Rump Length (CRL): The measurement of the length of the fetus from the top of its head (crown) to the bottom of its buttocks (rump). This method is considered the most accurate in the first trimester, offering an estimation with a margin of error of 3-5 days.
  • Head Circumference (HC), Abdominal Circumference (AC), and Femur Length (FL): In the second trimester, these measurements can also be used to estimate gestational age, though with slightly less accuracy than first-trimester measurements.

3. IVF and ART

For pregnancies achieved through In Vitro Fertilisation (IVF) or other Assisted Reproductive Technologies (ART), the exact dates of embryo transfer are known, providing a precise method of dating the pregnancy.

4. Developmental Milestones

In some cases, especially when LMP and ultrasound data are not available, healthcare providers may use specific developmental milestones observed during an ultrasound to estimate gestational age. However, this method is less accurate compared to direct measurements.

5. Physical Examination

In the early stages of pregnancy, a physical examination of the mother’s uterus can provide a rough estimate of gestational age. This method, however, is rarely used on its own due to its broad margin of error.

How accurate is the EDD estimation from ultrasound?

The accuracy of the Estimated Due Date (EDD) from ultrasound is a pivotal concern in prenatal care, influencing a wide array of clinical decisions and maternal expectations. Ultrasound-based EDD estimation is renowned for its precision, particularly when performed in the first trimester of pregnancy. This period is considered the golden window for EDD estimation through ultrasound due to the uniformity of fetal growth across pregnancies, making early ultrasounds the most reliable method for dating a pregnancy.

First-trimester ultrasounds, typically conducted before 14 weeks of gestation, can predict the due date with an accuracy of +/- 5 to 7 days. The crown-rump length (CRL), which measures the length of the fetus from the top of the head (crown) to the bottom of the buttocks (rump), serves as the cornerstone for this estimation. Due to the standardised rate of fetal growth during these initial weeks, the CRL provides a precise benchmark for calculating the gestational age and, subsequently, the EDD.

The accuracy of ultrasound-based EDD estimations slightly diminishes as the pregnancy progresses. In the second trimester, the margin of error increases to approximately +/- 7 to 14 days. By the third trimester, the variability in fetal growth rates among different pregnancies can lead to an accuracy range of +/- 2 to 3 weeks. Despite this gradual decrease in precision, ultrasound remains a crucial tool for EDD estimation throughout pregnancy, offering significant advantages over other methods such as the date of the last menstrual period, especially when the menstrual cycle is irregular or the conception date is uncertain.

Moreover, ultrasound examinations provide additional benefits beyond EDD estimation, including the assessment of fetal growth, detection of congenital anomalies, evaluation of the placenta, and monitoring of the pregnancy’s health. The integration of ultrasound in prenatal care exemplifies its indispensable role in modern obstetrics, facilitating informed clinical decisions, optimising maternal and fetal outcomes, and enhancing the antenatal care experience.

Use our Pregnancy Due Date Calculator to find out your EDD!

What if I already gone past my EDD as estimated by ultrasound or LMP, should I be worried?

Going past your Estimated Due Date (EDD) as determined by ultrasound or Last Menstrual Period (LMP) can be a source of concern for many expectant mothers, but it is a relatively common occurrence. It’s important to understand that the EDD is an estimate, not a guaranteed delivery date. In fact, only about 5% of babies are born on their precise due date. Most babies are born within a week or two before or after the estimated due date, with a significant number arriving safely after this period.

If you have gone past your EDD, it’s crucial not to panic. Here are several reasons why being past your due date is usually not a cause for immediate concern:

  1. Variability in Pregnancy Length: Every pregnancy is unique, and the length can vary. The EDD is based on a 40-week gestational period, starting from the first day of the last menstrual period, but this is an average rather than a rule.

  2. Accuracy of EDD: Both the methods of using LMP and ultrasound have their margins of error. Early ultrasound (in the first trimester) is generally more accurate than LMP for dating a pregnancy, but it’s still an estimate with a margin of error.

  3. Natural Variation: There is a natural variation in the length of pregnancy. It’s common for pregnancies to extend beyond 40 weeks, and most of these prolonged pregnancies result in healthy deliveries.

However, while it’s often not a cause for immediate concern, ongoing monitoring is essential to ensure the continued health of both the mother and the baby. Healthcare providers typically recommend increased surveillance if a pregnancy extends too far beyond the EDD. This can include:

  • Non-stress tests to monitor the baby’s heart rate and movements.
  • Amniotic fluid checks using ultrasound to ensure the baby still has enough amniotic fluid.
  • Biophysical profiles, which are more comprehensive exams that assess the baby’s breathing, movement, muscle tone, heart rate, and amniotic fluid levels.

In some cases, if a pregnancy extends significantly beyond the EDD (usually beyond 41 or 42 weeks, depending on the healthcare provider’s policies and the mother’s health), induction of labour may be suggested. The decision to induce labour is based on the balance between the risks of prolonged pregnancy (such as increased risk for stillbirth, decreased amniotic fluid, and the potential for a larger baby, which can complicate delivery) and the readiness of the baby and the cervix for birth.

It’s essential to maintain open communication with your healthcare provider if your pregnancy goes past your EDD. They can provide guidance, reassurance, and care tailored to your specific situation, ensuring the best outcomes for you and your baby. Remember, the goal is a healthy delivery, and sometimes that means waiting a bit longer for your little one to decide it’s time to make an appearance.

What is classified as pre-term birth?

Pre-term birth, classified as the delivery of a baby before the completion of 37 weeks of pregnancy, is a critical concern in the field of obstetrics and neonatal care. Unlike full-term pregnancies, which culminate after 37 to 42 weeks, pre-term births occur when a baby is born prior to the 37th week, marking a significant deviation from the expected gestational period. This early delivery can lead to various health challenges for the newborn, including respiratory issues, digestive problems, and developmental delays, highlighting the importance of specialised care and attention for these infants.

Pre-term birth is further categorised into three subgroups based on the gestational age at birth: extremely pre-term (less than 28 weeks), very pre-term (28 to less than 32 weeks), and moderate to late pre-term (32 to less than 37 weeks). The categorisation helps healthcare professionals in tailoring the care and interventions necessary for the optimal development of pre-term infants, addressing the unique needs based on the degree of prematurity.

The causes of pre-term birth are multifaceted, encompassing a range of factors such as genetic predispositions, maternal health issues (e.g., infections, chronic conditions like diabetes or high blood pressure), lifestyle factors, and complications in pregnancy like preeclampsia or placental problems. Preventive measures and early detection of risk factors play a crucial role in mitigating the incidence of pre-term births, underscoring the importance of comprehensive prenatal care.

Addressing the health challenges associated with pre-term birth requires a multidisciplinary approach, involving obstetricians, neonatologists, paediatricians, and a host of other healthcare specialists. The management and care of pre-term infants are tailored to their specific needs, ranging from respiratory support for those with underdeveloped lungs to specialised nutritional support to promote growth and development.

Pre-term birth remains a leading cause of neonatal mortality and morbidity worldwide, making it a significant public health issue. The ongoing research and advancements in neonatal care continue to improve the outcomes for pre-term infants, emphasising the importance of access to quality prenatal and postnatal care in reducing the prevalence and impact of pre-term births.

What is the first scan offered by the NHS?

Your first scan with the NHS is likely to be the Nuchal Translucency/Dating scan at 11-14 weeks. This scan is predominantly designed to screen for the same chromosomal anomalies as NIPT using an older methodology developed in the 1990s. NIPT is a far more advanced test developed in 2011 with a much higher accuracy.

Most crucially, majority of NHS Trusts have no protocols for structural anomalies screening until ~20 weeks. Our scans are specially designed to screen for every anomaly detectable from an early gestational age and will improve the management of the pregnancy.

Learn more about our 10 week scan. Use our pregnancy due date calculator to get the best possible dates for your scans!

What is the earliest scan I can have during pregnancy?

A Viability Scan, also known as an early pregnancy scan, is an ultrasound examination that is usually carried out between 6 to 9 weeks of pregnancy. This scan plays a crucial role in confirming the presence of your baby’s heartbeat, determining the number of babies you’re expecting, and estimating the gestational age of your pregnancy, helping to establish a more accurate due date.

What Happens During a Viability Scan?

During a Viability Scan, a small ultrasound device called a transducer is either passed over your belly or, more commonly in early pregnancy scans, used internally via the vagina to get a clear view of your womb. The procedure is safe, painless, and non-invasive. The internal scan does not harm the baby or interfere with the pregnancy.

Why Is a Viability Scan Important?

  1. Confirmation of Pregnancy and Heartbeat: One of the primary reasons for this scan is to confirm the pregnancy is developing inside the womb and to detect the baby’s heartbeat. This can provide immense reassurance to expectant parents.

  2. Determine the Number of Babies: Whether you’re carrying one baby, twins, or more can be identified during this scan.

  3. Assessing Pregnancy Viability: The scan assesses the health and viability of the pregnancy. It checks for any signs of an ectopic pregnancy (where the embryo implants outside the womb) and can help in identifying any early signs of potential complications.

  4. Estimating Gestational Age: The scan provides an estimation of how many weeks pregnant you are based on the size of the embryo. This helps in calculating a more accurate due date.

What Are the Outcomes?

Following a Viability Scan, you should have a clearer understanding of your pregnancy’s health status. Most parents find this early scan a comforting confirmation of their pregnancy’s progression. However, if there are concerns, such as signs of an ectopic pregnancy or lack of heartbeat, your healthcare provider will discuss the next steps with you, offering support and advice on how to proceed.

Do you want to know more or need a consultation?

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