Antenatal Care: Establishing Gestational Age with Ultrasound

00:01 So moving on to that ultrasound and establishing the gestational age.

00:06 Again using ultrasound the 1st trimester is the most accurate.

00:10 How we use it? We measure the crown-rump length.

00:13 So the distance from the crown of the fetus to the rump.

00:16 We expect that to be within one week of the last menstrual period.

00:21 So let me explain that in a little bit detail.

00:23 If by a patient's last menstrual period, the fetus is measuring 8 weeks and 2 days.

00:29 And by our ultrasound, the measurement is 6 weeks and 5 days.

00:34 We would refer to the ultrasound.

00:37 Last menstrual period again is only accurate if it's a 28 day cycle and it's regular.

00:43 The ultrasound is only going to tell us how far along the pregnancy is based on the date of conception.

00:49 So in that case we will go with the gestational age the ultrasound is giving us.

00:53 Let me give you another example.

00:55 If the last menstrual period says the patient is 7 weeks and 2 days, and our ultrasound says she's 7 weeks and 5 days, which one would we choose.

01:05 You guessed it right.

01:06 We would use the last menstrual period.

01:08 Because the last menstrual period in the ultrasound are within one week of each other.

01:14 Now, in the 2nd and 3rd trimester, we can still use that for establishing gestational age.

01:19 But it is not quite as accurate as we do in the 1st trimester.

01:21 In the 2nd and 3rd trimester we use the biparietal diameter which is the distance between the parietal bones in the head.

01:30 We use the abdominal circumference.

01:32 And we use the femur length to determine the gestational age.

01:34 Now here is a little rule.

01:36 In the first trimester, the ultrasound can be off by one week.

01:39 2nd trimester two weeks, third trimester three weeks.

01:44 So why the big difference.

01:45 Why does it increase as a gestational age increases.

01:48 Well there are many things that influence the size of the fetus as we get into 2nd and 3rd trimester.

01:53 Their fetal factors, their maternal factors, their placental factors.

01:58 Again as we discussed in the 1st trimester, the only thing that influences the gestational age is the date of conception.

02:07 So let's move to another question.

02:10 A 23 year old presents to the emergency room with complaint of left lower quadrant pain.

02:14 She states that the pain is 3 out of 10.

02:18 She states she has slight vaginal bleeding.

02:20 And she reports a positive pregnancy test approximately 6 weeks ago.

02:24 This is her first pregnancy.

02:26 She has no other medical problems or surgical history.

02:29 On physical exam, vital signs are stable and she is afebrile.

02:33 On sterile speculum exam, you see slight vaginal bleeding.

02:37 And then by manual exam, she has slight tenderness in the left lower quadrant.

02:41 Lab result show that her beta-hcg is 400 milligrams per decaliter.

02:46 On the transvaginal ultrasound, there is no gestational sac in the uterus, there is no free fluid in the pelvis.

02:52 And the adnexa are within normal limits.

02:55 So what is the next best step in her management? A. Admit the patient to the hospital and prepare her for surgery.

03:03 B. Reassure the patient that her pregnancy is normal and her pain is expected for her gestational age.

03:08 C. Proceed with medical management as patient most likely has an ectopic pregnancy.

03:13 or D. Repeat the beta-hcg and ultrasound in 48 hours.

03:17 The answer for this one is D.

03:19 Remember that we don't see anything on transvaginal ultrasound in the uterus, until the beta-hcg is at least 2000.

03:27 With this patient her beta-hcg was only 400.

03:30 So this may very well be a normal pregnancy.

03:32 However, may be an ectopic pregnancy.

03:35 We really just don't have enough information to tell.

03:37 So the correct answer is to repeat the beta-hcg in 48 hours.

03:41 Because we do expect it to rise by 50% if this is a normal pregnancy.

03:45 And then repeat the ultrasound.

03:47 Because we may be able to see something in the uterus by that time.

03:52 So just to recap.

03:54 This is the high-yield information that you need to know for this lecture.

03:57 Confirming pregnancy and establishing gestational age.

04:00 Pregnancy can be established by last menstrual period.

04:03 Urine or serum pregnancy test or ultrasound.

04:06 The last menstrual period is the most accurate for establishing gestational age if it's consistent with the transvaginal ultrasound.

04:14 If the last menstrual period is unknown, 1st trimester ultrasound is the most accurate way to establish the gestational age.

04:22 When the beta-hcg is greater than 2000 milligrams per decaliter, gestational sac should be seen on ultrasound.

04:29 And then remember that other little fact I told you.

04:32 In the 1st trimester, the ultrasound should be within one week of the last menstrual period.

04:36 In the 2nd trimester, two weeks.

04:38 And in the 3rd trimester, three weeks.