Pregnancy Dating and the Antenatal Schedule: The Evidence Behind the Tool

A 24-year-old woman comes to the antenatal clinic for the first time. She is pregnant for the first time, and she is sure of the first day of her last period. Her cycles have always been long, about 35 days. By her dates she is 21 weeks today. She has not had a scan. She asks you two questions: when is the baby due, and is everything all right? This is an illustrative case, not a real patient.

The Pregnancy & Antenatal Scheduler will answer the first question for you, and lay out every test, tablet and vaccine against a date. What it no longer carries is some of the why. The comparison of growth restriction with macrosomia, the precision of the scan charts and three trial results that sat in its Evidence tab have moved here, so that the tool stays quick at the bedside. By the end of this article you will know why the date matters so much, when a scan should change it, and when it should not.

1. One wrong number at booking moves everything

Why fuss over a few days? Because the gestational age decides when aneuploidy screening is worth doing, sets the legal window under the MTP Act, decides whether a woman in preterm labour is given steroids and magnesium, and settles when a post-term pregnancy is induced. One wrong number at booking moves all of them, in the same direction.

Gestational age is counted from the first day of the last menstrual period, not from conception. Naegele’s rule adds 280 days to that date. It assumes a 28-day cycle with ovulation on day 14. Our patient’s cycles are 35 days long, so she ovulates around day 21, and her pregnancy is 7 days less advanced than her dates suggest. The tool corrects for this: it adds the cycle length minus 28 days to the due date.

2. How the crown-rump length dates a pregnancy

An ultrasound in the first trimester, up to and including 13+6 weeks, is the most accurate method of dating, to within 5 to 7 days (ACOG Committee Opinion 700). The measurement is the crown-rump length, the straight-line length of the embryo from the top of the head to the bottom of the rump. The tool turns it into days with the INTERGROWTH-21st first-trimester equation (Papageorghiou 2014).

“Is the scan really that exact?” Nearly, early on, and less so as the embryo grows. The standard deviation widens with CRL, from about 2.7 days at 15 mm to about 4.2 days at 95 mm. The older Robinson and Fleming chart, still printed on many Indian and UK reports, differs by under a day across the same range (Robinson 1975). So when your report uses the older chart, you need not recalculate. What matters more than the chart is the technique: the mean of three measurements, in a true mid-sagittal plane.

Our patient has no early scan. At 21 weeks the scan dates her by composite biometry, the head, abdomen and femur measurements together, which is a less precise method.

3. When a scan should change the date, and when a small baby should not

Her scan measures 19+5 weeks. That is 9 days less than her dates. Should the due date move? The rule the tool applies is ACOG’s: the bigger the baby, the bigger the discrepancy needed before the date is changed. Between 16+0 and 21+6 weeks the date is changed only for a discrepancy of more than 10 days. Hers is 9, so her dates stand. Remember too that her long cycles already make her about a week less advanced than a 28-day calculation would say.

“Then why not simply believe the scan, every time?” Because a scan measures size, and size is not age. A baby who is small because it is not growing well will measure young. Redate it, and you convert a growth problem into a dating problem, and the one diagnosis that mattered disappears from the notes. This is the common mistake, and it is an understandable one: the machine prints a gestational age, and it looks more objective than the woman’s memory. Which is why a size and dates discrepancy is read as a growth question first and a dating question second.

This table used to sit in the tool beside that rule. It compares the two kinds of growth restriction with the large-for-dates baby. Read the “Effect on Dating” row first: it is the reason the table matters to dating at all.

The symmetrical, early-onset baby is the one most likely to be wrongly redated, because everything about it measures young. The macrosomic baby can pull the due date forward, which risks an induction that is earlier than it should be.

4. Three trials behind three lines of the schedule

Aspirin

If our patient had chronic hypertension, the tool would ask you to start low-dose aspirin, between 12 and 16 weeks and no later than 28. Where does that come from? ASPRE used 150 mg at bedtime from 11 to 14 weeks until 36 weeks and reduced preterm pre-eclampsia by 62 per cent (Rolnik 2017). American bodies write 81 mg because that is their tablet strength. In India the tablets are 75 mg and 150 mg, so write the strength on the prescription.

The DIPSI test

She has her DIPSI test at this first contact: 75 g of glucose by mouth, whether or not she has eaten, and a plasma glucose two hours later. Hers is 128 mg/dL. That is below 140, so it is not gestational diabetes, though FOGSI would call 120 to 139 gestational glucose intolerance and follow her up.

“So she is clear?” Not as clear as it sounds. Against IADPSG criteria, the non-fasting DIPSI test had a sensitivity of 22.6 per cent in 1031 women (Mohan 2014). A negative DIPSI is a weak negative. You may have seen a different sensitivity quoted for the same test. The figure of 27.7 per cent, often seen quoted for the same test, comes from the same paper: it is the sensitivity against the older WHO 1999 criteria rather than against IADPSG. Both numbers are Mohan 2014. Neither is from the WINGS study. Where a fasting oral glucose tolerance test with IADPSG thresholds can be done, it is the better test.

Counting the movements

Should she count kicks to ten every day? The AFFIRM trial, a stepped-wedge cluster randomised study of 409,175 pregnancies across 33 UK hospitals, found no reduction in stillbirth from a package of reduced-foetal-movement awareness and management (4.06 against 4.40 per 1000, adjusted odds ratio 0.90, 95 per cent CI 0.75 to 1.07), while caesarean birth rose from 25 to 28 per cent and neonatal unit admission rose (Norman 2018). So the tool asks her to report a change in the pattern of movement promptly, at any hour, rather than to count to a number.

Back to the clinic

Our patient leaves with a due date from her own dates, corrected for her long cycles, because the scan was within the 10-day limit for 21 weeks. If a later scan shows the baby measuring small, nobody will redate her: they will look for growth restriction. She has her tablets on a calendar, her next DIPSI at 24 to 28 weeks, a note that a negative result is a weak negative, and the instruction to report any change in her baby’s movements at once. Each of those is a line in the tool.

Try this

A woman conceived by in vitro fertilisation, with a day-5 blastocyst transfer. A scan at 10 weeks measures 6 days smaller than her transfer dates. The resident wants to move her due date. Should he? Illustrative case.

Answer. No. When the pregnancy comes from assisted reproduction, the date of transfer is known exactly, and the due date is the transfer date plus 261 days for a day-5 blastocyst. An ART-derived due date is not revised on the strength of a later scan. A baby measuring small against a known date is a finding to watch, not a reason to change the date.

Take-home points

• The gestational age at booking decides screening, the MTP window, steroids and induction; get it right once and protect it.
• Correct Naegele’s rule for the cycle length, and trust a first-trimester crown-rump length above all other methods.
• Change the date only when the scan differs by more than the limit for its gestation, and never redate a pregnancy dated by ART.
• A baby measuring small late in pregnancy is a growth question first; redating it hides the diagnosis.
• A negative DIPSI is a weak negative, and a change in movements is reported at once rather than counted.

Read with this: Postpartum Haemorrhage: The Evidence Behind the Pathway.

Declaration: the author built and maintains the tool. It is free to use. The figures in this article are those the tool carried in its Evidence tab on 4 November 2026, each from the source named beside it.

References

1. ACOG, AIUM and SMFM. Committee Opinion No. 700. Methods for Estimating the Due Date. Obstet Gynecol. 2017;129(5):e150-e154.
2. Papageorghiou AT, Kennedy SH, Salomon LJ, et al. International standards for early fetal size and pregnancy dating based on ultrasound measurement of crown-rump length in the first trimester of pregnancy. Ultrasound Obstet Gynecol. 2014;44(6):641-648.
3. Robinson HP, Fleming JE. A critical evaluation of sonar crown-rump length measurements. Br J Obstet Gynaecol. 1975;82(9):702-710.
4. ACOG and SMFM. Committee Opinion No. 579. Definition of Term Pregnancy. Obstet Gynecol. 2013;122(5):1139-1140.
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10. ACOG Committee Opinion No. 455. Magnesium Sulfate Before Anticipated Preterm Birth for Neuroprotection. Obstet Gynecol. 2010;115(3):669-671.
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19. Indian Thyroid Society and FOGSI. Recommendations for the Management of Thyroid Dysfunction in Pregnancy. Mumbai; 2019.
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21. ICOG and FOGSI. Recommendations for Good Clinical Practice: Use of Anti-D Immunoglobulin for Rh Prophylaxis. 2009; and FOGSI, Anti-D Immunoglobulin for Rh Prophylaxis: Key Practice Points. 2022.
22. Government of India. The Medical Termination of Pregnancy (Amendment) Act, 2021 (No. 8 of 2021) and MTP (Amendment) Rules, 2021, Rule 3B.
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25. Norman JE, Heazell AEP, Rodriguez A, et al. Awareness of fetal movements and care package to reduce fetal mortality (AFFIRM): a stepped wedge, cluster-randomised trial. Lancet. 2018;392(10158):1629-1638.
26. Doubilet PM, Benson CB, Bourne T, Blaivas M. Diagnostic criteria for nonviable pregnancy early in the first trimester. N Engl J Med. 2013;369(15):1443-1451. Reaffirmed by the Society of Radiologists in Ultrasound first-trimester lexicon, Radiology 2024;312(2).