Last Menstrual Period Dating
How to Calculate Pregnancy Due Date
Pregnancy is a time of joyful waiting and excitement. Expectant parents want to know when to expect their new little bundle of joy so they can prepare mentally and physically for childbirth and prepare a safe home for the baby. Obstetricians want to know the due date so they can monitor fetal growth, which can be an indicator of problems with the pregnancy. The last menstrual period is used to determine the approximate age of the pregnancy. Ultrasound is often used to confirm the due date and to monitor growth.
Last Menstrual Period (LMP)
The age of a pregnancy is determined by the first day of the last menstrual period. Gestational age includes the weeks of menstruation, follicular development, and ovulation, which is when conception occurs. For this reason, gestational age can also be referred to as menstrual age. It should be about two weeks longer than the fetal age, which is the actual age of the fetus.
The following is an example of last menstrual period dating and how it compares to fetal age:
Last menstrual period: January 1
On February 12 (six weeks after the last menstrual period), the following would be true:
Gestational age / Menstrual age: 6 weeks
Fetal age: 4 weeks
Gestational age is commonly used while fetal age is not.
Menstrual Cycle Variations and Pregnancy Due Date
The pregnancy due date can be easy to calculate because pregnancy calculators are widely available on the Internet. Many of these are combined with ovulation calculators. For many women, however, these calculators are not accurate.
Most women do not have a 28-day menstrual cycle. Some have periods every three weeks while others may have them as far apart as five weeks. Ovulation occurs at different times for these women. Generally, those with shorter menstrual cycles will be given a gestational age that is farther along than their LMP suggests, and those with longer menstrual cycles will be given a gestational age that is earlier than their LMP suggests. This is because ovulation occurs at different times for those with shorter and longer cycles. Some women have menstrual cycles that vary in length, so an ultrasound may be necessary for calculating the pregnancy due date.
Ultrasound
When the first day of the last menstrual period is unknown, or the lengths of the menstrual cycles are varied, ultrasound will most likely be used to determine the age of the pregnancy. First trimester ultrasound is done to acquire a crown-rump length. The ultrasound machine determines the due date and gestational age based upon the crown-rump length. Second trimester ultrasound is used to acquire measurements of the head, abdomen, and femur bone to determine an approximate due date, gestational age, and weight. This method becomes less accurate as the pregnancy progresses.
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