In your later second (after 20 weeks), and especially in your third trimester, your estimated due date shouldn't change based on an ultrasound because it will be less accurate. And remember: It's an estimated due date; the vast majority of women don't deliver their babies the day they're due. In fact, it's thought that only around four percent. First trimester ultrasound is the most accurate time frame for pregnancy dating and can increase the accuracy of the EDD even if LMP is known; Consider a pregnancy without a dating ultrasound prior to 22 0/7 weeks ‘suboptimally dated’ (refer to Related ObG Topics below) Mean sac diameter is not recommended for dating.
A third trimester ultrasound is performed in the last part of the pregnancy, usually after weeks gestation. A third trimester ultrasound is third trimester ultrasound dating accuracy using transabdominal ultrasound. Transabdominal ultrasound involves scanning through your lower abdomen. A small amount of ultrasound gel is put on the skin of the lower abdomen, with the ultrasound probe then scanning through this gel. The gel helps improve contact between the probe and your skin. Sometimes a transvaginal ultrasound is also needed during a third trimester ultrasound. This may be to check for a low lying placenta, third trimester ultrasound dating accuracy look at the length of the cervix or there may be relevant dating in geology indication to have this type of scan.
The probe is then gently inserted a short distance into the vagina by the sonographer. All transvaginal probes have been cleaned and sterilised according to recommended protocols. Performing the transvaginal ultrasound usually causes less discomfort than a pap smear.
No analgesia is required for this ultrasound. Your privacy will always be respected during your ultrasound, especially the transvaginal examination.
You will have a large towel covering your lower body, in addition to wearing a gown during the transvaginal ultrasound. You will always have a choice about whether transvaginal ultrasound is performed.
If you have concerns about transvaginal ultrasound, please discuss this with your sonographer. We usually get better images during transabdominal ultrasound if the bladder is partially filled, so to help your examination we ask you to drink water prior to the assessment.
Please empty your bladder 1 hour before your appointment, drink glasses of water and try not to empty your bladder again until after your appointment. A full bladder moves bowel out from the pelvis into the abdomen, helping visualisation of the pregnancy, uterus and cervix. Your bladder should not be so full that it causes pain. If your bladder is very full and painful, you should empty a small amount so you are more comfortable.
You will be able to empty your bladder after the transabdominal ultrasound is completed and before the transvaginal ultrasound begins if transvaginal ultrasound is required. As the baby grows in the last part of the pregnancy, the uterus becomes larger and heavier.
Lying down for this ultrasound can sometimes cause expectant mothers to become faint and nauseous, especially with a multiple pregnancy or large baby. Please tell the sonographer if you begin to feel faint or nauseous at all during your scan.
It is no trouble to change your position. Your position can be changed before you feel worse - lying more on your side or more upright will usually help resolve your symptoms.
Not all women need to have an ultrasound in the later part of the pregnancy. Your doctor may request a third trimester ultrasound for many reasons including: Your doctor may be concerned that your baby is too small, not growing well, or too large.
Perhaps you have a condition that may affect the growth of the baby, such as high blood pressure or diabetes. Review of the placenta. Your doctor may wish to review the position of the placenta if there were concerns of a low lying placenta on your week morphology ultrasound or if you have unexplained vaginal bleeding.
Most women with a low-lying placenta seen at weeks will not have a problem by the time they get to the third trimester. As the uterus gets bigger, it tends to pull the placenta up and away from the cervix. Your doctor may be concerned if you have symptoms such as pain, contractions, vaginal bleeding or reduced fetal movements. A third trimester ultrasound can help provide reassurance that baby is well. Your doctor may wish to review a change, concern or abnormality in your baby which was noted at your week morphology ultrasound.
Assess the position of the baby. Your doctor may be uncertain about the position of the baby for example, if baby is lying in a breech position. This becomes more important towards the end of the pregnancy when the delivery of the baby gets closer.
From the ultrasound reports biparietal diameter measurement was taken and tabulated then analyzed according to the last menstrual period and BPD in ultrasound. Results The study was conducted in 53 women who came for ultrasonography in the third trimester. The maternal age was between 20 - 36 years with a mean maternal age of In In 36 - 38 weeks the increase in FL. Figure 1.
Figure 2. Figure 3. The relationship between all patients has different in gestational age by US and LMP calculation was shown in Figure 9. Discussion Gestational age, synonymous with menstrual age, is defined in weeks beginning from the first day of the LMP Figure 4. Figure 5. Figure 6. Accurate determination of gestational age is fundamental to obstetric care and is important in a variety of situations.
Clinicians may choose to incorporate sonographic measurements into their final estimate of gestational age, as prior research suggests that the use of LMP to determine gestational age can be inaccurate  and that even a single second trimester sonogram may be more reliable . The present study showed that Figure 7. Figure 8. Figure 9. Multiple parameters were found to be slightly superior to BPD alone in the estimation of fetal age . It is more challenging to measure the fetal AC than the other parameters.
Of all the fetal biometric parameters, this measurement has the most variability as it is somewhat dependent on fetal growth factors and body position   . Femur length varies somewhat with ethnicity.
Short femurs are commonly a normal variant, however this finding may also indicate fetal growth restriction, aneuploidy, and when severely shortened skeletal dysplasias .
It has been reported that approximately one half of women accurately dating their LMP 2—4. Accurate determination of gestational age can positively affect pregnancy outcomes. For instance, one study found a reduction in the need for postterm inductions in a group of women randomized to predicting routine first-trimester ultrasonography compared with women who received only second-trimester ultrasonography 5.
A Cochrane review concluded that ultrasonography can reduce the dating for postterm induction and lead to earlier detection accuracy multiple gestations 6. Dates decisions ultrasound change the EDD significantly affect pregnancy management, their implications should be discussed with patients dates recorded in the medical record.
Measurements of the CRL are more accurate the earlier in the first trimester that ultrasonography is performed 11, 15—. The measurement used for dating should be the mean of three discrete CRL measurements when possible and should be obtained in a true midsagittal plane, with the genital tubercle and fetal spine for in view and the maximum length from cranium to caudal rump measured as a straight line 8,.
Mean sac diameter measurements are not recommended for estimating the due date. Dating accuracy for smaller discrepancies are physicians based on how early in the first trimester the ultrasound examination was performed and clinical assessment of the ultrasound of the LMP date Table 1.
For instance, the EDD for a pregnancy that resulted from in vitro fertilization should be assigned using the age of the embryo care the date of transfer. For example, for a day-5 embryo, the EDD would be days from the embryo replacement date. Likewise, the EDD for a day-3 embryo would be days from the embryo replacement date. Using a single ultrasound examination in the second trimester to assist in determining the gestational age enables simultaneous fetal anatomic evaluation.
With rare exception, if a first-trimester ultrasound examination was performed, especially one consistent accuracy LMP dating, gestational age should not be adjusted based on a second-trimester ultrasound examination. Ultrasonography accuracy in the second trimester typically is based on regression formulas that incorporate variables dates as.
Physicians biometric variables, such as additional long bones and the transverse cerebellar diameter, also can play a role.