Determining gestational age is one of the most critical aspects of providing quality prenatal care. Knowing the gestational age allows the obstetrician to provide care to the mother without compromising maternal or fetal status. It allows for correct timing of management such as administering steroids for fetal lung maturity, starting ASA therapy with a history of pre-eclampsia in previous pregnancies, starting hydroxyprogesterone caproate (Makena) for previous preterm deliveries.
Clinical History and Physical Exam
An average pregnancy lasts 280 days from the first day of the last menstrual period (LMP) or 266 days after conception. Historically, an accurate LMP is the best estimator to determine the due date. Naegele’s rule, derived from a German obstetrician, subtracts 3 months and adds 7 days to calculate the estimated due date (EDD). It is prudent of the obstetrician to get a detailed menstrual history including duration, flow, previous menstrual periods, and use of hormonal contraceptive. These factors are used to determine the length of her cycles and ovulation period. There are several fallacies with Naegele’s rule. First, a woman may not accurately recall the first day of her menstrual cycle. Second, this method assumes a woman’s cycle is exactly 28 days, with ovulation occurring at day 14, however, it does not consider menstrual cycles with shorter or longer durations. Third, there are small variations in duration between fertilization and blastocyst implantation. Last, this method cannot differentiate between menstrual bleeding and early pregnancy bleeding. Parikh’s formula was created to account for differences in menstrual cycle length. Parikh’s formula calculates EDD by adding nine months to the start of the last menstrual period, subtracting 21 days, then adding the duration of the previous cycle.
Other clinical methods that can calculate EDD include uterine size by pelvic examination using the over-the-curve technique. The over-the-curve technique involves using a measuring tape in centimeters from the superior aspect of the pubic symphysis to over the top of the uterine fundus. Highly trained practitioners can palpate the uterine fundus as early as 12 weeks, commonly at the symphysis. Clinical landmarks approximate a 20-week size gestation at the umbilicus, with a 16-week size gestation at the halfway mark between the symphysis and the umbilicus. The over-the-curve technique is most practical from 16 to 38 weeks with +/- 3cm margin of error. Even though this method is informative, it is subject to error secondary to anatomical and/or structural changes of the uterus. For instance, if a patient has uterine fibroids, this can decrease the accuracy of this technique. For patients who have had a previous cesarean, the uterus is generally in a higher position because of pelvic adhesions. Other factors which decrease accuracy include body habitus, multiple gestations, and a retroverted uterus.
Fetal heart tones are auscultated with the fetoscope at 19-20 weeks. Electronic Doppler device can detect fetal heart tones as early as 8-10 weeks gestation. It is important to understand all the clinical estimators of calculating due date, have a margin of error of +/- 3 weeks.
If a patient cannot recall the first the day of her last menstrual period, has irregular cycles, or Naegele’s rule cannot be used, the next step is ultrasonography. Ultrasonography is best within the first half of the pregnancy. Transvaginal (TV) sonogram can identify an intrauterine pregnancy (IUP) approximately 4 weeks after the LMP. An embryo is usually seen later between 5-6 weeks. Mean sac diameter (MSD) can be used early in gestation to estimate due date if there is no embryo on the sonogram. To calculate the MSD, the diameter sac measurement must be in the perpendicular plane, with a mean of three measurements. Calcualtion of EDD is from MSD by adding 30mm. MSD increases by 1mm per day during early pregnancy. Given the large margin of error. MSD should not be used for final due date estimation. Additionally, anembryonic pregnancies can have a measurable MSD.
One should not use MSD if you can calculate the crown rump length (CRL). CRL dating is the best method from 8 6/7 to 13 6/7 weeks. CRL is the longest straight-line measurement of the embryo from the outer margin of the cephalic pole to the rump. An accurate CRL includes mid-sagittal plane, genital tubercle, and fetal spine in a straight line with a margin of error +/- 5-7 days. An average of three measurements estimates the gestational age and EDD.
For second trimester sonography, between 14 0/7 weeks and on, CRL is no longer the most accurate form of measurement. Instead fetal biometrics including, biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL) estimate gestational age and EDD.
The BPD measurement criteria include; an oval head shape instead of round head shape, midline structures should be centered, measured at the level of the thalamus and cavum septum pellucidum, should not include top orbits. The BPD measurement is from the outer edge of the proximal skull to the inner edge of the distal skull. The HC measurement criteria should include all those required for the BPD and is measured around the outer perimeter of the skull. The AC measurement criteria include; the abdomen should be round, not oval shaped or squashed, true transverse image, images of ribs should be symmetrical on both sides, measured at the level where the umbilical vein joins the portal sinus, calipers should go all the way to the skin surface. The FL measurement criteria include; perpendicular to the direction of insonate, ends should be sharply visible, and measurement should exclude epiphyses.
An official EDD is established, after calculating the first-trimester sonogram EDD date and then using the LMP. If the LMP and first trimester EDD are within 7 days of each other, the LMP estimates the due date. The margin of error is reduced depending on when (i.e., how early) the sonogram occurred. If the sonogram was before 8 6/7 weeks and LMP is within 5 days, then the LMP estimates the due date, otherwise use the sonographic EDD. If a sonogram took place after 14 0/7 weeks and if the EDD is within 14 days of the LMP, then the LMP estimates due date. If the sonogram did not happen until the third trimester, then EDD can be calculated by LMP, if the LMP is within 21 days. The American College of Obstetrics and Gynecology (ACOG) deems dating sonograms dated after 22 weeks suboptimal for GA measurement; this is because before 22 weeks GA, fetal biometry parameters are accurate within +/- 7days to the CRL in the first trimester. After this range, variation in fetal size growth makes this less precise. If multiple sonograms exist, the EDD from the earliest sonogram detecting GA becomes the EDD.
The same rules as discussed above apply to twins or higher order gestations. If there is size discordancy between the twins, the larger twin is used to calculate the EDD.
Assisted or artificial conception
The due date is determined by dates of ovulation, egg retrieval, insemination, cleavage stage or blastocyst transfer. Assisted reproductive technology (ART) use the principle of Naegle's rule as the foundation for estimating the due date. If fresh in vitro fertilization (IVF) is done, EDD is calculated by adding 266 days to egg retrieval/fertilization. If using a frozen embryo (day 3), EDD is 263 days from date of embryo transfer to account for three days of embryo culture. If a day 5 blastocyst is implanted, add 261 days to this date to calculate EDD.
The determination of gestational age is vital to providing quality prenatal care.
Establishing correct EDD is paramount in coordinating care between healthcare professionals that include a nurse, laboratory technologists, a pharmacist, and a variety of physicians in different specialties. This will allow timely laboratory testing since most of the tests are time-sensitive in pregnancy.
Having accurate birth dating might decrease maternal/fetal morbidity and or mortality through timely consulting with experts in the field of maternal-fetal medicine, obstetrics/gynecology, oncology, or genetics. For example, fetal genetic abnormalities can be detected in a timely fashion providing the mother with sufficient time to make a lifetime decision.