Calculating Gestational Age in IVF Pregnancy
In IVF pregnancies, calculate gestational age by adding 14 days to the number of completed weeks since fertilization (oocyte retrieval or embryo transfer date). This provides highly accurate dating since the exact fertilization date is known. 1
The Standard IVF Dating Method
Add 14 days to the date of oocyte retrieval or embryo transfer to establish the gestational age, as this accounts for the typical time from last menstrual period to ovulation/fertilization in natural conception. 1
This method is recommended by international consensus guidelines for assisted reproductive technology pregnancies because it provides a known fertilization date, eliminating the uncertainty inherent in last menstrual period-based dating. 2
Express gestational age as weeks and days (e.g., 15 weeks 5 days) rather than rounded weeks for optimal accuracy. 2
The Ultrasound Dating Controversy in IVF
While the standard approach uses the known fertilization date, emerging research suggests ultrasound crown-rump length (CRL) measurement may still be valuable even in IVF pregnancies:
Studies show that ultrasound-based gestational age (GAUS) is consistently 1-3 days ahead of IVF-calculated gestational age (GAIVF), with the discrepancy likely due to anticipated ovulation and fertilization timing in IVF cycles rather than accelerated embryo development. 3, 4, 5
One study found CRL-based dating was 1.9 days shorter than IVF dating in singletons and 1.4 days shorter in twins when using second-trimester measurements. 6
Another study demonstrated a systematic overestimation of 3.0 days when using current CRL reference charts compared to true IVF gestational age. 4
Practical Clinical Approach
Despite the known fertilization date, consider the following algorithm:
Use the IVF-calculated date (fertilization + 14 days) as the primary dating method per consensus guidelines. 1, 2
Perform first-trimester ultrasound with CRL measurement between 8-13 weeks as you would in any pregnancy for confirmation and to rule out anomalies. 2
Document both the IVF-calculated date and ultrasound-measured date in the medical record. 2
If discrepancy exists between IVF date and CRL measurement:
- Small discrepancies (1-3 days) are expected and likely reflect biological variation in IVF cycles. 3, 4
- Consider using the ultrasound date if the discrepancy is ≥5 days, as this threshold is used in natural conceptions to adopt CRL-based dating. 2, 7
- This approach may help re-align IVF pregnancies with natural conceptions for screening test interpretation. 3
Critical Considerations for Screening Tests
Accurate dating is essential for prenatal screening because biochemical markers change rapidly with gestational age—maternal serum AFP increases 10-15% per week in the second trimester. 2, 7
Using IVF-calculated dates ensures consistency with the known biological timeline, but be aware that screening test reference ranges were typically developed using ultrasound-dated pregnancies. 2
Never average the IVF date and ultrasound date—choose one method and use it consistently for all clinical decisions. 2, 7
Common Pitfalls to Avoid
Don't assume that knowing the exact fertilization date eliminates the need for first-trimester ultrasound—this scan serves multiple purposes beyond dating, including confirming viability, number of fetuses, and early anomaly detection. 1, 2
Be cautious when interpreting screening results if there's a discrepancy between IVF and ultrasound dating, as even small dating errors can significantly affect biochemical marker interpretation. 2, 7
For multiple gestations from IVF, perform first-trimester dating when CRL is 45-84 mm to simultaneously assess chorionicity, dating, and nuchal translucency. 2