Do you know how to calculate your due date (estimated date of confinement)? It is extremely important that you and your prenatal care provider use whatever information is necessary to accurately calculate the due date for your delivery.
Establishing an accurate due date is the most important piece of clinical information you will determine at your first prenatal visit.
Your due date will factor into many decisions that will be made throughout the course of your pregnancy.
Here are some of the diagnoses and management decisions that are affected by your due date:
- Screening Tests
- Preterm Labor Decisions
- Management Decisions if you go past your due date.
- Fetal Growth Evaluation
- The timing of Delivery if Complications Arise
To Calculate Your Due Date, You Will Be Asked Detailed Information About Your Period
You may be asked the following questions about your menstrual cycle at your first prenatal visit. This information is used to calculate your due date. This information may or may not be necessary to calculate your due date. It will depend on how regular your periods are (every 28 days) and if your clinical findings are consistent with your due date based on the first day of your last menstrual period. Your due date will be most accurate if your LMP is correct. as it is reflective of when you ovulated during the month.
WHAT WAS THE 1ST DAY OF YOUR LAST MENSTRUAL PERIOD (LMP)?
This helps determine what day you ovulated if you have a regular 28-day menstrual cycle. Ovulation is when the egg is released from the ovarian follicle and occurs on Day 14.
Here are some useful facts about your menstrual cycle:
- A 28-day menstrual cycle is divided into a proliferative phase (the 1st 14 days) and a luteal phase (the last 14 days).
- During the proliferative phase (also known as the follicular phase), ovarian follicles in the ovary start to mature. Each follicle has an egg and ultimately one follicle will be chosen to ovulate and release an egg on day 14. The proliferative phase is also the time when the menstrual lining is being hormonally prepared for implantation.
- The luteal phase starts with ovulation and ends with the first day of your next period (unless pregnancy occurs).
- If the egg is fertilized in the fallopian tube (during an ovulatory cycle), it will be transported into the uterine cavity to implant on the uterine wall.
ARE YOU CERTAIN ABOUT THE 1ST DAY OF YOUR LMP?
It is extremely important that you are certain about this date as it will be used to date your pregnancy, assuming your periods are regular and other findings noted on your exam or through testing determines that the date is incorrect.
WHAT WAS THE DATE OF YOUR PRIOR PERIOD?
This helps establish that you have had recent regular cycles.
WAS YOUR LAST MENSES NORMAL IN INTERVAL AND DURATION?
Again, this can help determine the accuracy of your LMP.
WHAT IS YOUR USUAL MENSTRUAL INTERVAL (DAY 1 – DAY 1)?
This helps to determine how reliable your period is for establishing a due date. If you have very consistent 28 day periods that you have regular midcycle ovulation.
AT WHAT AGE DID YOU START MENSTRUATING (MENARCHE)?
Menses can be irregular when they start (menarche) and when they end (menopause). This information can also help to determine how accurate your last menstrual period is.
DID YOU TAKE ANY PREGNANCY TESTS? WHAT DATE?
If accurate, it is the best indicator of when you first documented you were definitely pregnant.
DO YOU KNOW WHEN YOU GOT PREGNANT?
Though fertilization can happen for a few days after intercourse.
DO YOU HAVE A HISTORY OR HAVE YOU BEEN TREATED FOR INFERTILITY?
Infertility is associated with irregular or delayed ovulation. If treated then due date will have been established.
WERE YOU USING HORMONAL BIRTH CONTROL WHEN YOU GOT PREGNANT?
- Hormonal birth control can interfere with your period and this can make your last menstrual period unreliable.
- If your menstrual cycle is not reliable, it may be necessary to use blood tests (HCG) and ultrasound to establish your due date. Determining your estimated date of confinement using these tests is most accurate early in the first trimester.
- Once your due date is determined early in your pregnancy, it will not change since there will be no tests for dating that will be more accurate. As noted below, ultrasound evaluation for determining your due date gets less accurate as you pregnancy advances.
Ultrasound to determine your estimated date of confinement:
1st Trimester: (especially within the first 12 weeks)
An ultrasound done under 12 weeks of pregnancy will give the most accurate assessment of your due date. The reason for this is because the ultrasonographer is able to see significant image changes over a short period of time. In contrast, an ultrasound done during the third trimester of pregnancy is very inaccurate because the fetus is fully developed and changes in imaging are very subtle.
The main reason first-trimester ultrasounds are so accurate to calculate your due date is that there are very early milestones that start to appear. These findings can be accurate to within 5 days.
This accuracy of 5 days can vary based on:
- The quality of equipment used
- The experience of the sonographer
- Whether the ultrasound is transvaginal or transabdominal. (Almost all early ultrasounds are now done transvaginally.)
The specific findings on a 1st-trimester ultrasound are listed below. They offer information about dating and fetal well-being:
- Gestational Sac: The gestational sac can be seen as early as the 4th week of pregnancy. However, it is not uncommon to become visible between the 5th and 6th week. This is the earliest finding to confirm that the pregnancy is developing in the uterus. Often times this ultrasound finding is compared to a blood test called an HCG (Human Chorionic Gonadotropin). It has been determined that when this blood test is above 2500 an intrauterine sac should be visible. If not there may be no problem because the laboratory measurement can vary and if one has twins the levels of HCG will be higher. The most important reason your provider will want to confirm an intrauterine pregnancy once your HCG reaches 2500 is to exclude the possibility of having an extrauterine pregnancy. This is a pregnancy that is developing but is not implanted in the uterine cavity. Pregnancies that implant outside the uterine cavity are called ectopic pregnancies. Most often ectopic pregnancies are located in the fallopian tube. It is very important to diagnose an ectopic pregnancy as soon as possible so treatment can be initiated before the pregnancy enlarges and ruptures out of the tube. This can lead to significant blood loss and even death if not diagnosed and managed properly.
- Fetal Heart Rate: Ultrasound detection of a fetal heartbeat can be detected as early as 5 1/2 weeks. However, it is not uncommon to not see a heartbeat before 6 or even 7 weeks. The reasons for this are many including some that have been mentioned.
- Yolk sac and Fetal Pole: These findings are indicative of a developing embryo within the gestational sac. The yolk sac serves as a source of nutrition until the placenta takes over. The fetal pole is the developing embryo. These structures can start to be seen with transvaginal ultrasound imaging around 51/2 to 61/2 weeks. There are reasons why these structures may not be visible until later. It can often be a technical limitation of the ultrasound exam. However, it is important to note that there is a condition called a blighted ovum where the gestational sac is empty and a yolk sac and fetal pole never develop.
- Crown-Rump length: After the gestational sac is visualized, this is another parameter used to calculate your due date. It can be used for this purpose if it is measured between 7 and 12 weeks. Extensive data has been collected to correlate a crown-rump measurement of the embryo with gestational age.
2nd Trimester within 2 weeks
3rd Trimester within 3 weeks
How to calculate your due date based on a regular 28-day menstrual cycle:
Subtract 3 months from the 1st day of your last menstrual period and add 7 days.
*This formula assumes you have regular 28-day cycles.
So what data may be necessary to calculate your due date? (Including information you provide and laboratory tests.)
To calculate your estimated date of confinement accurately a combination of the following information will be necessary:
- Last Menstrual Period – The first day of your last menstrual period.
- Pregnancy Test HGC – This test measures the level of human chorionic gonadotropin (HCG). It is used to determine if you are pregnant and if the pregnancy is
This article is for educational purposes only, your prenatal care provider will definitively determine your due date based on the information you report to your provider before or at your first prenatal visit. Call the office if you have any concerns regarding your pregnancy (pain, bleeding, etc).
**Ectopic pregnancies (a pregnancy outside of the uterus: tube, ovary, etc.) are very serious. If this diagnosis is made by your provider you will be treated accordingly.
Other articles you may find helpful:
- 10 Must-know Tips for Your First Prenatal Visit
- 9 Questions You Must Ask At Your First Prenatal Visit
- 10 First Trimester Pregnancy Tips You’ll Be Glad to Know