What is the Importance of Dating Scan in Pregnancy?

MedPiper Technologies and JournoMed had conducted a World Preeclampsia week Webinar Series from 23rd May to 29th May in association with National IRIA Preventive Radiology and Samrakshan Committees. On Day 3, Dr. Akanksha Bhagel spoke about the importance of Dating Scan and Dr. Anjali Gupta and Dr. Lalit Sharma reviewed some case studies in relation to dating scan.

How to detect and confirm Pregnancy?

Pregnancy can be detected with a pregnancy test kit available in the market. Women can opt for the test if they doubt that they are pregnant. A few pregnancy kits don’t give accurate results for pregnancy. So the next option which doctors prefer is the ultrasound scan or dating scan which reveals the state of pregnancy and helps with estimating the due date of delivery. Dr. Akanksha Bhagel, spoke about the importance of Dating Scan which is performed during the first trimester of the pregnancy. Dating scan helps with the following:

• To rule out abnormalities of molar pregnancies

During the first trimester scan, it is very essential to take care of the foetus and the embryo surroundings. Doctors can easily recognize if the pregnancies lead to a complete molar or partial molar situation. In partial molar pregnancies, abnormalities are seen. Only a dating scan can indicate the molar pregnancies.

• To confirm the location of ectopic pregnancy

If a pregnant woman suffers from severe pain in the lower abdomen, there are chances of ectopic pregnancy. A clear examination reveals the location and can help doctors detect ectopic pregnancy. The location of ectopic pregnancies can be abdominal, ovarian, cervical, interstitial, tubal (isthmus and ampulla), infundibular or peritoneal.    

• To confirm viability 

There are certain criteria which doctors can use to confirm if it is a failed pregnancy or a viable pregnancy. In a failed pregnancy, the Crown rump length (CRL) is greater than 7mm, there is no constant heartbeat and no alpha fetoprotein in the review scan i.e. greater than 2 weeks with an empty gestational sac. In failed pregnancy, there is an empty amnion, gestational sac and the enlarged yolk sac greater than 7mm. 

• To determine the number of foetuses, assess chorionicity and amnionicity

Doctors can easily detect the no. of foetuses, assess chorionicity and amnionicity with the help of the ultrasound scans. The scans can also help determine doctors if the pregnancy is monozygotic or dizygotic twins. Based on the ultrasound scan report, doctors can also check for markers such as t-sign, lambda sign or a twin peak. 

How to recognize the gestational age accurately?

By convention, the EDD (Estimated due date) is calculated after the first day of the LMP (Last Menstrual Period), assuming a regular menstrual cycle of 28 days and the ovulation date will be on the 14th day. 

This method is not applicable because:

• Inaccurate recall of LMP observed in 50% of women
• Irregularities in cycle length: 40 % of women receive first trimester ultrasonography had their EDD adjusted to a discrepancy of greater than 5 days between the ultrasound dating and the LMP dating 
• Viability observed during the time of ovulation

Hence, Ultrasound is the most reliable method to establish pregnancy dating, especially the first trimester ultrasound.

Best period to date pregnancy

The best period to predict pregnancy date is between 11-13 weeks of gestation period and the crown rump length accurate value must be taken into consideration while measuring the estimated due date (EDD).  By observing the ultrasound scan, one can calculate the estimated due dates of the pregnancy.

There can be a discrepancy in the last menstrual period estimated due date and the crown rump length estimated due date. So the EDD value differs for both.

• If the crown rump length value is greater than equal to 5 days before 9 weeks, then it is the value of last menstrual period EDD
• If the crown rump length value is greater than equal to 7 days between 9 to 13 weeks 6 days, then the value is EDD discrepancy value

Note: To determine crown rump length, the foetus should occupy ⅔rd of the screen while scanning. The probe should be parallel in the position with the foetus and maximum length should be considered. The foetus should remain stationary, which gives a clear picture of the true sagittal section. 

Criteria for redating in second trimester

If a first trimester scan is not performed and one is unsure of the last menstrual period (LMP) or doesn’t have a corresponding LMP, one can go for a redating scan in the 2nd trimester.

Check points

While initial visit of a mother in the second trimester, doctor must examine 

• Preliminary medical report along with the scan report showcasing the crown lump length captured properly.
• Transcranial doppler, a form of ultrasound scan that detects the risk of stroke
• A follow up visit recommended for observing the growth pattern of the foetus
• Initial scan considered as date scan

What is seen in a Date Scan Report?

One septum, thylami , outer margin of the skull, femur length- end to end, end point of diaphysis, femur measurement, cross section of abdominal circumference at the end, the outer circumference of the abdomen are visible: Cerebellum and diaphysis remain invisible

Dating Scan for Twins:

In cases of spontaneous conception, there might be a discrepancy in the crown rump length of the larger twin

Dating at 3rd Trimester:

Dating scan at 3rd trimester is suboptimal and it is not an assigning scan. The overall foetal growth environment must be monitored.

Important points to be remember in dating scans

• The mean sac diameter is not recommended to assess the gestational age.
• The decisions to change the estimated due date (EDD) needs to be discussed with the patients and should be recorded
• Crown rump length needs to be recorded properly
• 11-14 weeks scan, an integral part of the scan must be performed by doctors
• No composite biometry at third trimester and always mention a centile and an attached growth chart 
• Mentioning how the gestational age is assigned also taken into consideration 
• An elective delivery must be avoided in suboptimal dating pregnancies 
• Never change the date once fixed to second or third trimester  

Some of the other uses of dating scans include:

To rule out the early pregnancy complications

Generally, after the 2nd and 3rd trimesters, it is very difficult to observe the surroundings of the foetus. The woman may face early pregnancy complications such as threatened, inevitable, incomplete, complete and missed abortion (types of spontaneous abortion). 

With the help of dating scans, one can rule out the spontaneous pregnancies or spontaneous abortions. Ultrasound can provide a clear picture of the womb from which the doctors can predict the condition in advance and take care of the pregnant woman. 

To assess uterine environment- adnexa, ovarian mass and fibroids

Adnexa is a lump of tissue mass present in the uterus. Dating scans are prescribed to a pregnant woman to assess the uterine environment. The doctors can clearly observe if there is any lump, mass, adnexa or fibroids formation in the uterus, which sometimes can be a life threatening situation. Consulting a gynaecologist can help one in resolving the issue.

First trimester aneuploidy screening 

The first trimester aneuploidy screening performed by nuchal translucency sonography which is also one of the most effective methods to detect Down’s syndrome. This screening is performed during the 10-14 weeks of gestation. Also, performing the uterine doppler at the first trimester scan helps to check for organ formation and alignment. It also helps to check the symmetry of the upper and lower limbs, skin integrity and the continuity of the spine.

Biochemical Screening 

Biochemical screening is a set of pregnancy tests where doctors look for hormones like human chorionic gonadotropin (hCG) in the maternal blood. hCG is a biochemical marker that indicates pregnancy. The incidence of preterm births and stillbirths can be reduced by performing biochemical screening procedures. 

Interventional decision making

Dating scan is important for interventional decision making. During interventional decision making doctors must consider chorionic villus sampling and amniocentesis. The chorionic villus sampling timing is of 10-13 weeks and amniocentesis is 15-22 weeks. In amniocentesis chromosomal/genetic abnormalities or structural birth defects can be found. One can terminate the pregnancy if any genetic abnormalities are observed.

The known carrier status for a genetic condition such as hemoglobinopathies, sickle cell disease, neurofibromatosis, cystic fibrosis, muscular dystrophy etc can be detected. Family history of genetic condition with an advanced paternal age, parental aneuploidy or aneuploidy mosaicism, placental carrier of chromosomal arrangement can be clearly indicated.

To prevent preterm births, post term births and related morbidities:

By using the dating scan, radiologists can prevent the preterm births, post term births and the related morbidities that evolve during the time of pregnancy.

Importance of dating scans via Samrakshan Program

Only one dating scan is preferred by the government guidelines. Many times, pregnant women are unaware of LMPs and it is an emergency situation to recognize the foetal growth. The Samrakshan program highlights the importance of dating scan concepts where the main focus would be on structural anomalies or the genetic marker evaluations. But the statistics of Samrakshan showcase that foetuses are lost due to congenital anomalies , abnormal foetal growth or feto maternal environment. To avoid the abnormalities, the dating scan of the foetal growth is very important. Doppler parameters provide a clear understanding of the foetal growth.

Dating scan beyond 26 weeks 

The dating beyond 26 weeks is preferably avoided, the patient presenting late and the previous scan has no common reports. The best gestational age for dating scan is 11-13 weeks. Once the estimated due date has been generated by LMP or by CRL, subsequent changes in the scan dates can lead to complications during the delivery such as preterm births and perinatal complications. Early dating scans must be preferred to detect the foetal growth and feto maternal environment.

Dating scan provides the actual gestational period. It is necessary to spread awareness and health education about dating scans and thus, provide care to pregnant women and avoid mortalities.