Pregnancy following IVF: how to calculate the due date and gestational age

In a natural pregnancy, the due date is calculated from the first day of the last period, and the first-trimester ultrasound scan corrects this if necessary. Following IVF, there is no guesswork involved: the clinic’s report states the date of transfer and the embryo’s stage. However, you still need to know how to do the calculation, and understand why your Greek clinic and your maternity hospital do not always give the same date.

The principle: a ‘virtual first day of menstruation’

By convention, the gestational age is counted in weeks of pregnancy, from the first day of the last period, which is approximately two weeks before fertilisation. After IVF, this date is reconstructed theoretically:

• the day of fertilisation corresponds to the day of egg retrieval; this is ‘day 0’ for the embryo;
• the date of the virtual period is 14 days before fertilisation;
• a blastocyst transferred on day five (D5) therefore corresponds to a pregnancy of 2 weeks’ gestation and 5 days on the day of transfer; a D3 embryo corresponds to 2 weeks’ gestation and 3 days.

The American College of Obstetricians and Gynaecologists (ACOG) has derived a simple rule from this: for a pregnancy achieved through IVF, the due date is calculated based on the embryo’s age and the date of transfer, i.e. 261 days after the transfer of a Day 5 embryo and 263 days after the transfer of a Day 3 embryo.

Calculation on a case-by-case basis

If you know the date of the egg retrieval instead, the calculation is the same: the due date falls 266 days after the egg retrieval, the date of fertilisation.

Example. A Day 5 blastocyst transferred on 10 March 2026 corresponds to a virtual last menstrual period on 19 February 2026. The due date at 40 weeks’ gestation falls on 26 November 2026, i.e. 261 days after the transfer.

For a frozen embryo transfer, what matters is the stage of the embryo when it was vitrified, not the date of the egg retrieval: a Day 5 blastocyst frozen three years ago and transferred today results in a pregnancy of 2 weeks and 5 days as of today.

The first few weeks, in days after transfer

This table, applicable to a Day 5 blastocyst, helps you keep track of the time between transfer and the first ultrasound scan.

What you should see at each stage is explained in the article on the first ultrasound scan, and how to interpret hCG levels in the article on beta-hCG.

Why the ultrasound scan does not change the due date

In a spontaneous pregnancy, the first-trimester dating scan may result in the due date being shifted by several days, because the date of the last menstrual period is uncertain and ovulation can vary. Following IVF, the date of fertilisation is known: ACOG recommends dating the pregnancy based on the IVF data, rather than on the ultrasound measurement. An embryo that is slightly smaller or slightly larger than expected is within the normal range; it is up to the doctor to assess whether a significant deviation warrants monitoring, not to recalculate the due date.

Two countries, two dates: 40 weeks’ gestation or 41 weeks’ gestation

This is the most common source of confusion amongst my patients. Most countries set the due date at 40 weeks’ gestation, i.e. 280 days after the last menstrual period. France conventionally uses a presumed due date of 41 weeks’ gestation, one week later. Both calculations start from the same point and are correct; they simply do not use the same convention. A French patient may therefore be given a due date of 26 November by the Greek clinic and a due date of 3 December by her maternity unit, for the same pregnancy.

There is nothing to worry about, but it is worth noting that maternity leave, late-pregnancy appointments and decisions regarding induction are based on the conventions of the country where you will give birth.

What you need to provide to your doctor

Keep the transfer report and hand it over at your first antenatal appointment. It must state the date of transfer, the embryo’s stage (D3, D5 or D6), whether it was fresh or frozen, and the number of embryos transferred. With these four pieces of information, any doctor, in any country, can determine the same gestational age; the due date then depends on the standard – 40 or 41 weeks’ gestation – of the country where you are receiving care. Arrangements for follow-up care upon your return are detailed in a separate article.

If your report is in Greek or if any of this information is missing, please write to me: I will obtain it from the clinic.

Frequently asked questions

Source: https://fertilitegrece.com/en/blog/ivf-due-date-calculation/ — Triada Baloukoudis, IVF patient coordination in Greece. General information, not medical or legal advice. Printed on

Triada Baloukoudis
Independent IVF patient coordinator in Thessaloniki, with 24 years of experience supporting patients travelling from abroad. More · Contact · Facebook