Methods Doctors Can Use to Calculate Due Date

Plenty of legends have been told about delivery day dramas, from surprise arrivals on the way to the hospital to grand entrances delayed for days. As it happens, only around 4% of expecting moms deliver their babies on target with their originally estimated due date.

The roughest way to estimate your baby’s due date is to tack 40 weeks onto the date of your last menstrual period. But, enhancing outcomes for both fetal and maternal health depends on accurate due date prediction. This makes knowing how providers calculate due dates a crucial part of the prenatal picture.

Whether you’re reviewing the math on your future baby’s due date or just curious why “crown-rump length” is so important, we’ll cover the methods used to calculate due dates so that you can count down the days with confidence.

How do doctors calculate due date?

Due dates are typically set when you receive your first ultrasound exam—generally around your 12-week mark. Some healthcare providers recommend ultrasounds as soon as 7 weeks into pregnancy, though these are considered very early ultrasounds.

First ultrasounds are intended to:

• Confirm a pregnancy by listening for your baby’s heartbeat
• Check for multiples
• Take your baby’s fetal measurements
• Calculate your baby’s due date

No matter what method they use, your healthcare provider’s due date calculation is always an estimate. The American College of Obstetricians and Gynecologists recommends three main ways of arriving at that estimated due date (EDD).

Using Ultrasound Dating

Most embryos and fetuses grow at the same rate in early pregnancy. Knowing this, a doctor or a healthcare provider can use your baby’s measurements to estimate when you’ll go into labor up to 13 6/7 weeks into your first trimester.

The main fetal measurement used is CRL, or crown-rump length, which measures from the top of the head to the tailbone. CRL is usually quite accurate, estimating the correct due date within a 5- to 7-day range. However, several conditions must be in place to predict your baby’s due date accurately:

• Fetal position – Sonographers must be able to see your baby at a certain angle to take an accurate measurement. If your baby is in an angled position, CRL measurements may be distorted and thus inaccurate.
• Fetus size – It’s more difficult to precisely measure CRL the further along you are into pregnancy. If you’ve booked your first ultrasound testing any time after the first trimester (the final week is your thirteenth), the healthcare professional won’t be able to use CRL to reliably predict the exact date of delivery.

Sometimes, there may be a discrepancy between your unborn baby’s CRL length and the date of your last period. If this happens—whether because of an inaccurate answer or irregular menstrual cycles—your doctor will side with the EDD provided by your ultrasound.

Using Your Last Menstrual Period

The date of your last menstrual period (which physicians shorten to LMP) is typically treated as a supporting metric for due date calculation. The ACOG discourages from using it exclusively to estimate due date for two reasons:

1. It’s been suggested that up to ¼ of reproductive-aged women experience some degree of irregularity in their menstrual cycles.
2. While the accepted standard for cycle length is 28 days, many women have shorter or longer cycles. Normal cycle lengths fall anywhere within the range of 21 to 35 days.

Several conditions and circumstances can further contribute to cycle length irregularities, including:

• Reproductive conditions like PCOS, endometriosis, or PID
• Lifestyle factors like stress, weight gain or loss, or getting sick
• Reproductive choices like taking hormonal contraceptives (e.g. “the pill”)
• Use of certain medications, such as steroids

In general, the incidence of menstrual irregularity tends to increase as women age, often overlapping with the period when many consider starting families or having children. This is one reason why booking your first ultrasound is so important for ensuring the most accurate possible estimated due date or EDD prediction.

Using Your IVF Transfer Date

More than 1 million babies have been born via assisted reproductive technologies (ART) like IVF since the technology was first used in 1987. If you’ve conceived your baby through IVF, several factors might be considered to determine your EDD:

• Your last date of ovulation
• When your eggs were retrieved
• When insemination occurred

Most healthcare providers use your transfer date—when your blastocyst or embryo was embedded—to determine your estimated due date.

Another method is to simply add 266 days to your date of egg retrieval or fertilization. If your embryo was cryopreserved, they’ll add between 261 and 263 to that date to determine your EDD (depending on when it was transferred).

Clinicians don’t commonly use a baby’s date of fertilization because IVF transfers take place so soon after the egg is fertilized. Most transfers are scheduled between 3 and 5 days after fertilization, whether or not your embryo was stored through cryopreservation.

Understanding Gestational Age

Clinicians define gestational age as the length of time between when a baby is conceived and when they will be born.

Most pregnancies themselves span 280 days dating from a woman’s last menstrual cycle. However, like a pregnancy, gestational age is measured by the week. At the time of a healthy baby’s birth, the gestational age should fall between 38 and 42 weeks.

Due date calculations and gestational age are closely related, but they’re not quite the same thing. The ACOG states that methods for determining gestational age—the most reliable being an ultrasound—are critical for securing pregnancy outcomes for both mom and baby. Use of ultrasounds can help limit:

• The need for inducing labor post-term
• The need to alter EDDs later in pregnancy

Why would an EDD be subject to change?

Do due date changes happen? Second ultrasounds are administered in the second trimester, which starts at the 14th week of pregnancy. They tend to be most accurate at the start of your second trimester (between weeks 14 and 21), though they’re typically given between 18 and 22 weeks.

In most cases, the gestational age estimated by your first ultrasound will be confirmed at the time of your second. However, because second-trimester ultrasounds include an anatomy scan that measures different parts of the fetus (like its head circumference, femur length, and abdominal circumference), discrepancies between EDDs can occur.

That said, only in rare cases do healthcare providers or OBGYNs alter their original EDD—even if your second ultrasound indicates your baby may arrive at a different date. Can’t wait for your appointment? Use our due date calculator to find out how far along you are and when you can expect your new baby to arrive! The main caveat is if the date of your last menstrual period is thought to be inaccurate. In this case, your provider may switch your baby’s EDD.

How important are EDDs for pregnancy?

The importance of EDD estimation can’t be overstated, and many would say it’s the single most important element in managing your pregnancy.

Getting an accurate EDD gives OBGYNs the ability to:

• Administer tests and screenings at the correct time
• Gauge the rate of your unborn baby’s development with accuracy
• Preempt possible complications that could arise at certain trimesters during pregnancy
• Know whether or not to induce labor or administer a cesarean birth (these are not recommended for incorrectly or sub-optimally dated pregnancies)
• Account for possible complications, like having gestational diabetes, when establishing a birth plan

Remember, it’s incredibly common for your baby’s actual delivery day to miss the mark from your exact EDD—around 96% of babies arrive off-schedule.

Statistics provided by the CDC report that 1 in every 10 babies born in 2022 were pre-term (arriving before the 37-week mark). Preterm labor is more likely if you:

• Are carrying multiples (twins)
• Have a history of preterm labor
• Are sick with an infection
• Are an adolescent or over 35 years old
• Come from a lower-income household
• Belong to a racial or ethnic minority group
• Use tobacco or recreational drugs
• Are under a significant amount of stress.

Overdue pregnancies can also happen, whether or not your EDD was miscalculated. You’re more likely to have a late arrival if:

• You’re pregnant for the first time
• You’ve been pregnant before, and have a history of post-term births (pregnancies that last longer than 42 weeks)
• You have a higher-than-average BMI
• You’re having a baby boy

Other than dealing with several pregnancy symptoms by week, plenty of moms struggle with the elements of chance that come naturally with any pregnancy. But, by prioritizing your prenatal care and getting as much insight as you can about your pregnancy, you’ll be better prepared for whatever comes as you approach your due date.

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This post has been reviewed for accuracy by:

Sarah Cacia

Sarah Cacia, currently leading as the Director of Business Development at Gateway Genomics, parent company of SneakPeek, brings an impressive blend of bioengineering expertise and clinical business acumen. Her extensive experience, marked by roles at renowned institutions like Genentech and UC San Diego's Cardiac Mechanics Research Group, underpins her deep understanding of biotechnology and clinical research. A UC San Diego alumna with a Bachelor of Science in Bioengineering: BioSystems, Sarah's rich professional background empowers her to provide authoritative insights into the cutting-edge developments in the field.

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