Ultrasound Dating Calculator

A first scan often prints two ages. One comes from the first day of the last period. The other comes from the length of the embryo. Clinics in the United States are supposed to pick one estimated due date, talk it through, and write it down. Later scans confirm growth against that date. They are not a fresh vote every visit.

The age on both lines is gestational age, counted from a theoretical last period, not from conception. On the day an egg is fertilized, the clock already says about 2 weeks. An embryo that has existed for 6 weeks is called 8 weeks pregnant. That is why a “6-week scan” can show a sac and a tiny embryo, and why an IVF calendar feels two weeks ahead of the embryo’s real age. The due date is 280 days from that theoretical period: 40 weeks, not 38 weeks of embryonic life.

ACOG Committee Opinion 700, written with AIUM and SMFM and reaffirmed in 2022, picks the row from gestational age by last menstrual period. The word that matters is “more than.” Through 8 weeks 6 days, a gap of 5 days stays on the period date. Six days is the first gap that meets the rule. From 9 weeks 0 days through 13 weeks 6 days, the cutoff is more than 7 days. Exactly 7 stays.

The row follows the period age, even when the scan is the number that looks “behind.” A person who is 8 weeks 0 days by last period and 7 weeks 0 days on the scan is a 7-day gap in the “more than 5 days” band. The US rule would move the due date. The same 7-day gap at 10 weeks by last period would not, because that row allows more than 7.

The early visit most people call a reassurance scan is there to answer where the pregnancy is, whether an embryo is visible, and sometimes whether there is a heartbeat. A careful crown-rump length from that visit can date a US pregnancy. ACOG treats a first-trimester crown-rump length, up to and including 13 weeks 6 days, as the most accurate ultrasound clock, and it stops trusting that length once it passes about 84 mm, near 14 weeks.

NICE’s antenatal schedule, recommendation 1.2.14, offers a different scan: between 11 weeks 2 days and 14 weeks 1 day, to set gestational age, to see if there is more than one fetus, and, if you opt in, to screen. NHS Scotland will not lock an estimated due date from a crown-rump length under 45 mm. That length is brought back around 12 to 13 weeks. From 45 mm through 84 mm, the length itself sets the date. Past 84 mm, dating moves to head circumference.

Turn a millimeter length into weeks and you have to pick an equation. This page uses the Robinson formula adopted by the British Medical Ultrasound Society in 2009: gestational age in days equals 8.052 times the square root of 1.037 times the crown-rump length, plus 23.73. NHS screening froze that one equation so a 45 mm embryo is 11 weeks 2 days in every lab that follows the chart, and 84 mm is 14 weeks 1 day. The valid range here is 5 mm to 84 mm.

Many US machines print a Hadlock age instead. INTERGROWTH-21st, published in 2014, is a newer international standard for 15 mm to 95 mm. On a typical length it lands within a couple of days of Robinson. That gap is the formula. It is not a second embryo, and it should not be averaged with the clinic’s printed week. If both weeks and millimeters are on the report, the clinic’s weeks stay the headline above. The formula is the check for a millimeters-only report, and the explanation when the website and the printout are one day apart.

When two embryos are different lengths, dating from the smaller one makes the larger twin look big and can make a lagging twin look normal. AIUM and ISUOG both say a spontaneous twin pregnancy is dated from the larger crown-rump length in the first trimester, and from the larger head circumference once crown-rump dating is over. The size difference is still recorded. Dating from the larger twin does not mean the smaller twin is fine. It means the due date is not pulled backward by the smaller one.

That rule is for spontaneous twins. IVF twins use the embryo’s age and the transfer date, same as a singleton. Research has argued for the smaller length or the average, because in some IVF series the smaller twin sat closer to the known date. ISUOG looked at that and kept the larger length: switching would move most due dates by only a few days, and it would mark the larger twin as ahead. Type the larger length into this page if you are checking a spontaneous twin scan. Do not average the two millimeters first.

ACOG is direct about assisted reproduction: use the age of the embryo and the date of transfer. A day-5 embryo transferred today has the same due date whether it was frozen last week or last year. The freezer does not add gestational weeks. The embryo’s clock starts again at transfer, already part-way through, because a day-5 embryo is five days past fertilization and gestational age adds the conventional two weeks before that.

The arithmetic ACOG gives is 263 days from a day-3 transfer and 261 days from a day-5 transfer. The matching theoretical last period is transfer minus 17 days for day 3, and transfer minus 19 days for day 5. That is the calendar this page builds when you pick an IVF anchor. The late-ovulation story does not apply. You are not guessing cycle day 14. If the scan still misses that known date by more than the cutoff, the calculator calls it a certain date that disagrees. It does not relabel the gap as a late period.

With a transfer date, “behind” is no longer a calendar mistake. The embryo’s age is known. A short crown-rump length against that age has been studied as a risk marker. It is still not a diagnosis you can make from a website, and this calculator will not produce one.

One Fertility and Sterility series looked at 940 IVF pregnancies scanned between 7 weeks 0 days and 8 weeks 6 days. After that scan, the miscarriage rate in the whole group was 12.7%. When the length was under the 10th percentile of a menstrual chart, about 41% of those pregnancies miscarried. When the length was not small, about 95% did not. The association was stronger at older ages in that paper: about 30% at age 30 when the length was small, and about 61% at age 45. The same study did not find a link from that small length to preterm birth or low birth weight. Those figures describe one cohort. They are not the chance sitting in your chart.

A 2025 paper in Reproductive BioMedicine Online showed the other half of the story. Pregnancies that were still going at the 11-to-14-week scan had crown-rump lengths from 1 mm to 17 mm on the day that was 49 days by the transfer clock. Same transfer date, a very wide range of lengths, and those pregnancies reached the dating scan. In that series, an empty sac with no embryo at or after 47 days by the transfer date did not continue. The authors said the finding needs confirmation before anyone treats it as a rule. A sac-only visit still belongs with the follow-up rules in the measuring-behind guide, not with a due-date tool.

ACOG Committee Opinion 688 calls a pregnancy suboptimally dated when no ultrasound has confirmed or revised the due date before 22 weeks 0 days. That is the situation this page is trying to keep you out of. A crown-rump length in the first trimester, compared with a period, ovulation, or transfer date, is how a US record gets a date that later decisions can trust.

If that early scan never happened, Committee Opinion 688 is specific. There is no role for an elective delivery chosen for convenience. Indicated deliveries still use the best clinical estimate, because there is no better clock to switch to. When age is uncertain, delivery is indicated at 41 weeks by that estimate: an unset date can mean the pregnancy is already further along than anyone thinks. Starting fetal surveillance at 39 to 40 weeks may be considered. A repeat ultrasound 3 to 4 weeks after the first one can support the working age, and it can also catch slowed growth. A routine amniocentesis just to test lung maturity is not part of that decision.

Read the repeat scan as a growth check. A fetus that measures small weeks after the dating window is not offered a new due date to match its size. The date, uncertain as it is, stays the best estimate. The size is a separate question.