 Today we are going to talk about a very basic but important topic in fetal ultrasound which is estimation of gestational age and assigning a correct delivery date. Accurate dating is very important for timing of appropriate ANC care for scheduling and interpretation of different ANC tests, determining growth which may be less or more. For the intervention purposes like deciding about pre-term fetus, post-datism, planning for early delivery etc. which are the correct methods or good methods of determination. LMP of course whenever the patient comes to you for the ultrasound the first thing we ask the patient is the LMP. But many a times the patients are not very good at remembering the LMP. It may be a delayed conception, periods may be coming late. So anyway the LMP is not a very good method of dating that pregnancy. If it is the IVF pregnancy it is a very accurate dating from that date of embryo transfer. It is in plus minus one day range. If the date of ovulation is known as in follicle studies or IUI is known then it is in plus minus three days range. In the first trimester CRL is the measurement which is used to determine the gestational age and in the second trimester it is a different parameters like BPD, HC, FL and AC which are used for determination of gestational age. So typically when the patient comes to you for the ultrasound in the patient data entry system in that page LMP is the first thing which we feed in and then we get a gestational age. We do a scan, you do the biometry and then you get the average ultrasound age. So early pregnancy is the most accurate for dating which is by CRL and after 14 weeks it is the different measurements what we take. So whenever the gestational age matches the ultrasound biometric age we assign the EDD as per the LMP. When the gestational age mismatches the ultrasound biometric age we assign the EDD as per the ultrasound age. So what are these criteria of mismatch? At what discrepancy we should reassign that you date. Is it different at different gestational age? So we will see that. So this chart tells us which is the acceptable range of variation between the LMP and ultrasound age. So they have divided into less than 14 weeks and more than 14 weeks for the important purpose of method of measurement which is the CRL before 14 weeks and after that it is the different measurements. Before 14 weeks it is subdivided into 2 groups less than 9 weeks and more than 9 weeks. So if the pregnancy is less than 9 weeks and if the discrepancy is in that 5 days range then it is acceptable. But if you find that your CRL is more than 5 days different than your gestational age by LMP then it is not acceptable and you should reassign the EDD. If the pregnancy is between 9 weeks to 14 weeks then the 7 days difference is acceptable. Beyond 14 weeks it is divided into 14 to 16 weeks, 16 to 22 weeks and beyond 22 weeks and the difference which is in that acceptable range is 7 days, 10 days and 14 days. But of course beyond 22 weeks it is not a good time to reassign the gestational age or to do a dating scan. So at early pregnancy USG at the dating scan the delivery date can be assigned very well. If discrepancy is less than variation expected at that gestational age then you should assign the EDD as per LMP, once assigned do not change the EDD in subsequent reports. If the discrepancy is more than variation expected at that gestational age then assign the EDD as per ultrasound. Make the mother aware about the difference, stress the importance of carrying the previous reports for subsequent scans. So we will just quickly see one or two examples. Now as you see here the discrepancy between the gestational age and CRL is just 3 days. So our chart tells us that when it is less than 9 weeks the acceptable difference is 5 days which is 3 days here. So what should I do? The EDD is assigned as per LMP. But in this case if you see the discrepancy is 6 days between the CRL age and the gestational age by LMP. So what does my chart tells me before 9 weeks the 5 days difference is acceptable which is more here so what should I do? I should in fact reassign the EDD as per ultrasound what the machine tells me as per the ultrasound age and which is different than this EDD which was 2812 but now it is 3rd of January. Now when I see the patient for the first time at around 12 weeks and if the discrepancy is 6 days what does my chart tell me? It is at 11 weeks 6 days. So this is falling between 9 to 14 weeks I will assign it only by CRL and the difference which is acceptable is 7 days. Here the difference is 6 days so what should I do? I will write the EDD as per LMP and will not change the EDD. Now for whatever reasons if the patient has come to me only at around 19 weeks 6 days there is no dating scan and now if I find the discrepancy which is around 10 to 12 days then what should I do? My chart tells me that if the discrepancy is 10 days or more I should reassign the EDD. So now what is the next step? I should not be in a hurry to reassign the date I will always look and ask the patient whether there is an early dating scan done and whether it is available. If it is there I will go back and check that report if it is not there then what should I do? What should I average the ultrasound age? I should look at the parameters. If all the parameters are similar then I will average the ultrasound age. If they are not similar and if they are different I will look at the different parameters. If the AC is smaller BPD, HC, FL are in the normal range or maybe sometimes the FL and HL are in the smaller size but BPD, HC are normal size then I will consider the possibility of growth disturbance, tort infections, chromosomal problems, skeletal problems etc. And then I should ask for a repeat ultrasound scan, I should look at the growth velocity, I should look at the other telltale signs of say infections of any chromosomal abnormalities and then take the action accordingly. Now look at this typical report. The discrepancy is of course more than 10 days. So now if I see the parameters, the gestational age by LMP is 19 weeks 4 days, BPD and HC are actually 19 weeks but my AC and FL are falling short. So in this situation I should not average out and say that there is an 18 weeks pregnancy and the discrepancy of 11 days is there and I should reassign the EDT as the patient gives the history of delayed periods but I should wait, I should look, I should rule out the other causes and then reassign the EDT. So in the second trimester if the parameters are similar then average out the parameters, if they are different then averaging is not recommended. In third trimester unfortunately you have missed the train and it is a wrong time to assign the age. Avoid these mistakes of giving different EDTs in the subsequent scans according to USG. Giving two EDTs in the report, you should never write EDT by LMP is so and so, EDT by ultrasound is so and so. You should assign only one EDT. Use only CRL to date the pregnancy in first trimester and not the gestational sack. Now if you see typically this report that the LMP tells you it should be 8 weeks, 3 days. You are taking gestational sack measurement which is 8 weeks, 2 days but if you see the CRL which is quite small and then the machine is averaging out the two measurements and telling you that this is 8 weeks and you get a spurious feeling that there is a difference of only 3 days and you will not reassign this EDT but if you really check the CRL is the best method and there is a difference of 6 days, there is a mismatch of 6 days and actually you should be reassigning this EDT by ultrasound age and this should be avoided. So whenever the patient comes for a falloff scan, you should look at the previous report. Look at if there was any discrepancy in the previous report and then you should enter the EDT in your machine and not the LMP. So typically if you see here that you enter the LMP and you have got the gestational age and the EDT. Now if you go through the previous reports and it tells you that the EDT which was assigned is actually different than what the machine tells you, then now you should not enter the LMP but what you should do is enter the expected date of delivery and leave this column blank and then it tells you that this is actually 21 weeks, 1 day pregnancy and not 22 weeks, 6 days. So you will not falsely call this pregnancy as a growth restricted baby. So the impression in the first trimester will typically look like this. Because there was a growth discrepancy of more than 10 days, you will write it as delayed conception and you have assigned the delivery date by the ultrasound age and only one EDT is given in that report. So whenever the next time the patient comes to you, you are not going to enter the LMP, you are going to write only according to the previous dating scan and now you will write in the bracket, expected by dating scan and not by expected by LMP and assigned delivery date by dating scan is so and so. So you should always remember that dating scan is best done between 6 to 10 weeks using CRL, you should assign the delivery date cautiously and once assigned, don't change the EDT in your subsequent reports, don't give two different EDTs in your reports, something like this should be avoided. Before you start doing follow up scans, always look at the dating scan only then you will be able to correctly diagnose the growth disorders in your subsequent scans. The AGA, HGA, FGR, LGA, all these terms you will better understand once you have understood the concept of dating scan and assigning the correct EDT. So in the next video, we will look at how to pick up these fetal growth disorders.