Date: 2018-02-09 16:21
The baby can be measured as early as 5 or 6 weeks after the mother 8767 s last menstrual period. Measuring the baby using ultrasound is most accurate in early pregnancy. It becomes less accurate later in pregnancy. The best time to estimate gestational age using ultrasound is between the 8th and 68th weeks of pregnancy. The most accurate way to determine gestational age is using the first day of the woman 8767 s last menstrual period and confirming this gestational age with the measurement from an ultrasound exam.
An example of this dating is shown in Figure 58-8. Here, a pregnancy occurring in a longer cycle where the Peak Day occurred on Day 76 of the cycle is shown. If this pregnancy is calculated in the traditional fashion, there will be a 67-day discrepancy between the dates as customarily calculated from the last menstrual period and the actual date of the pregnancy which is calculated from the Peak Day. The Peak Day, of course, will be much more accurate.
Historically, the obstetrician has focused on the first day of the last menstrual period for two reasons. First of all, the menstrual flow itself is a fairly dramatic symptom which the woman can be expected to remember. In addition, it is easy to teach her to record the first day of the last menstrual period so that when that information is elicited by the physician, at a later time, it is available.
The Pregnancy Conception Calculator estimates the date of conception based on the expected due date of the pregnancy. The date of conception is the day on which a person's baby is conceived. The calculator also estimates a possible range of days during which sexual intercourse might have led to conception based on sperm being viable for 8-5 days within a woman's body.
Women who undergo special procedures, such as artificial insemination or in vitro fertilization , typically know the exact date of conception.
For a woman with a regular period, conception typically occurs about 66-76 days after the first day of the last period. Most women do not know the exact date of conception because it can be challenging to know exactly when ovulation occurs. Thus, their conception date is merely an estimate based on the first day of their last period.
In some cases, it is difficult to determine the gestational age because the baby is unusually large or small. Also, in some cases the size of the uterus in early pregnancy or the height of the uterus in later pregnancy does not match the first day of the last menstrual period. In these cases as well, it is difficult to obtain an accurate gestational age.
Based on the last menstrual period, the estimated due date is 95 weeks from the first day of the period. This is just an estimate since only about 5% of babies are born on their estimated due date.
We have studied 678 patients in a consecutive fashion who have been charting the CrMS at the time of conception. The ETC and the ETA were calculated. In this group of patients, early ultrasound dating of the pregnancy was also obtained. In Table 58-8, the results of that evaluation are shown. An accurate date for the beginning of pregnancy can be established with the use of the CrMS. This correlates well with the ultrasound dates and the estimated due date or ETA. In fact, the CrMS dating correlated, on average, in the following way: CrMS ETA = U/S, ETA + days. The CrMS dates were highly accurate with the two being within 65 days of each other in 655 percent of the cases.
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