How to compute for aog using lmp?

A typical pregnancy lasts, on average, 280 days, or 40 weeks—starting with the first day of the last normal menstrual period as day 1. An estimated due date can be calculated by following steps 1 through 3:

For example: Your last menstrual period began on September 9, 2010. Counting back 3 calendar months would be June 9, 2010. Adding 1 year and 7 days would bring you to June 16, 2011, as your estimated due date.

This 3-step method is referred to as Naegele's Rule and is based on a normal 28-day menstrual cycle. Therefore, dates may have to be adjusted for longer or shorter menstrual cycles.

You may also estimate your delivery date by using the steps 1 and 2 and the chart:

The Current Date is set to your local workstation date. EDC by LMP is calculated by adding 280 days (40 weeks) to the first day of the last menstrual period. Gestation by LMP is calculated from the first day of the last menstrual period.

Other documented benefits of obstetric ultrasonography include the detection of multiple pregnancies and fetal abnormalities, and the identification of placenta previa.

In the early first trimester, when no structures are visible within the gestational sac, GA may be estimated from the sac diameter. Several formulas can accomplish this. A common method is to measure the mean sac diameter (MSD), by calculating the mean of the 3 sac diameters. GA is then determined by consulting a table. An alternative simpler method is to add 30 to the sac size in millimeters, to give GA in days. By the time the embryo becomes visible on ultrasound the sac diameter is no longer accurate in estimating gestational age.

If cardiac activity can be detected but the embryo is not measurable, the GA is about 5.5-6.0 weeks.

GA in the first trimester is usually calculated from the fetal crown-rump length (CRL). This is the longest demonstrable length of the embryo or fetus, excluding the limbs and the yolk sac.

The correlation between CRL and GA is excellent until approximately 12 weeks' amenorrhea. No sex or race differences are appreciable, but maternal characteristics, such as age and smoking, may have a significant effect beyond 10 week' gestation. The GA estimate has a 95% confidence interval of plus or minus 6 days, and it is most accurate between 7 and 10 weeks' amenorrhea. The following formula allows the estimation of GA (weeks) from the CRL (mm):

GA = –0.0007 (CRL)2 + 0.1584 (CRL) + 5.2876

Transabdominal ultrasonography may underestimate gestational age by an average of 1.6 days compared with transvaginal ultrasonography.

In twin pregnancies, the CRL of the smaller fetus is more accurate in determining gestational age.

The biparietal diameter measured between 9 and 13 weeks’ gestation has recently been shown to be at least as accurate as the CRL, with smaller random errors.

A study by Skupski et al developed a new formula to estimate gestational age from biometric data on ultrasound. The new formula was: GA = 7.85-0.127 * biparietal diameter + 0.07304 * head circumference + 0.00638 * abdominal circumference + 0.122 * femur length + 0.000685 * biparietal diameter * abdominal circumference - 0.00015 * head circumference * abdominal circumference.

Fetal biometry in the second trimester can yield acceptably accurate estimates of GA from 12 to approximately 22 weeks of amenorrhea. Recent work has shown that the accuracy of ultrasonographic biometry at 12-14 weeks' gestation is at least as good as biometry performed after 14 weeks. The best parameters are the biparietal diameter (BPD) and the head circumference (HC), which are virtually linearly related to GA.

The femur length (FL) can also be used and is nearly as accurate as head measurements.

Racial differences in FL are significant, but differences in HC are not. GA estimates by the BPD or HC have a 95% confidence interval of plus or minus 8 days. The following formula allows estimation of GA (days) from the BPD (mm):

GA = 39.1 + 2.1 (BPD)

Combinations of multiple variables to date a pregnancy have also been described; however, the improvement in accuracy from these algorithms is clinically negligible.

Fetal biometry in the third trimester is subject to much greater individual size variations than in the second trimester. Its accuracy for GA assignment is reduced considerably, and estimates may have confidence intervals of plus or minus 3 weeks. More recent work with pregnancies resulting from in vitro fertilization suggests that third trimester scans are considerably more accurate than previously thought, with random errors (1 SD) of 8-9 days. This is equivalent to confidence intervals of about plus or minus 2 weeks.

The table below illustrates the 95% confidence limits for GA estimated from the BPD in the third trimester.

Confirmation of fetal maturity may also be obtained by examining the ossification centers. The distal femoral epiphysis appears at a mean age of 32-33 weeks' gestation, but may be seen as early as 29 weeks' gestation ; its size increases linearly with gestational age. A distal femoral epiphysis diameter greater than 7 mm indicates a gestational age greater than 37 weeks. Visualization of the proximal tibial epiphysis indicates a gestational age of at least 35 weeks. The sonographic detection of the proximal humeral epiphysis has been correlated with a mature amniocentesis lung profile.

By knowing the first day of your last period, or your last menstruation day, you can figure out how far along you are in your pregnancy and what month you are in. With this information, you can also estimate your due date.

Through ultrasound, the doctor can also determine how far along you are. Fetal and uterine measurements can accurately relay the baby's age and estimate the delivery date.

Enter the first day of your last period below to calculate how far along you are in weeks and months, and when you can expect your baby to be born:

Sometimes it is not possible to know when your last period was, and in these cases, the doctor can order a dating ultrasound. My measuring uterine height, the ultrasound technician or doctor can calculate how far along you are as well as your estimated due date. Oftentimes, an ultrasound and a blood test to check beta-HCG levels are ordered together to determine dating.

To calculate how many weeks you are, start by taking note of the first day of your last period on a calendar, or your LMD (last menstruation day). Every 7 days from that first day counts as 1 week in your pregnancy.

For example, if the first day of the last period was March 11 and you recently had a positive pregnancy test, your first day of pregnancy is March 11 (and not the day of conception, or when you had sex). If March 11 was day 1, then March 18 (a week later) will mark week 1 of your pregnancy. April 16, for example is calculated as 1 week and 1 day of your pregnancy, and you will have been in your second month.

Calculation is done this way because the exact fertilization day is difficult to determine. Sperm cells can survive in the woman's body for up to 7 days, and can fertilize the egg and initiate the pregnancy on any of these days.

To determine how far along you are, you can reference our table below

Usually a pregnancy lasts 40 weeks, but the baby can be born between 39 and 41 weeks without any problems. If labor does not start spontaneously until the 41-week mark, the doctor may choose to induce labor with IV oxytocin.

To calculate the probable due date, which should be around 40 weeks after your LMD, add 7 days to your LMD, and then count 3 months back and add one year.

• 2 + 7 = 9.
• 11 – 3 = 9.
• 2021 + 1 = 2022.