Once in vitro fertilisation (IVF) has been carried out and the embryos are obtained, in some cases we might have several embryos to transfer. The objective is to try to achieve pregnancy as soon as possible, and to do this, we must select the embryo that presents the best prognosis as our first option.

Even though viable embryos with a poorer prognosis might still give rise to a pregnancy, because they are slightly less likely to give this outcome, they are not prioritised when transferring.

To select the best embryo for transfer, different classifications and filters are used:

• Morphology: due to their shape, the number of cells, their symmetry, their size … their appearance in general, embryos are catalogued in terms of quality from A to D (with A being the best quality). These qualities indicate the expected rate of pregnancy based on studies of embryo quality.
• • Morphokynetics: through the use of Time Lapse technology, not only can we see the appearance of the embryos at a specific moment to classify them in terms of quality; we can also observe them continuously (on video), which gives us much more information and allows us to distinguish very relevant and significant events in the development of the embryos. These events can differentiate embryos from each other, helping us to prioritise those with a better prognosis.
• Euploidy: by performing a biopsy of the embryo and subsequent genetic analysis, we can determine its state of euploidy, which means that it does not present any chromosome anomalies, in other words, it presents the normal number of chromosomes.
• Non-invasive genetic analysis: in cases where embryo biopsy is not indicated or recommended, there is the possibility of performing a similar test, but in a non-invasive way. The study is equivalent, although it does not affect the embryo because the sample is taken from the medium in which it is located. As such, the result once again indicates an order of priority based on the prognosis of pregnancy that each embryo presents, depending on its genetics.

A question that raises many doubts with patients and even some professionals is determining the date of delivery after assisted reproductive treatment.

There are more and more variants in assisted reproduction treatments: Artificial insemination, IVF with transfer on day 2, day 3, day 5, day 6, transfer of frozen embryos in substituted cycles (with oestrogens and progesterone), transfers in natural cycles, etc…

For hundreds of years, only the period of time between the last menstruation and childbirth was known. In fact, it was thought that fertilisation occurred during menstruation. This period of time is exactly 280 days (28 days x 10). If we want to use the Gregorian calendar (in which all months except February alternate between 30 and 31 days), delivery occurs about 9 months + 5 days after the start of the last period. This has remained unchanged since the dawn of time.

Since fertilisation actually occurs on the day of ovulation, in women who have longer cycles (ovulate later) or shorter cycles (ovulate earlier) calculating the due date based on their period is not suitable.

The calculation of the due date is much more reliable when we count 266 days from fertilisation or ovulation. But how many women know exactly when they have ovulated?

Women who undergo assisted reproduction techniques know exactly the day of ovulation since it coincides with the day of egg retrieval, fertilisation, or insemination. In that precise instant, we can truly begin to count the time.

Women who do not know the date of ovulation (266 days before delivery) usually count from the only date they know, which is the date their last period began. In this case, they must count 280 days, by adding to the 266 days of gestation the 14 days that usually occur between period and ovulation (provided that their cycles are regularly 28 days). In the case of irregular cycles, it should be calculated based on the measurement of the embryo on the first ultrasound.

It should not be counted from the date of the transfer, as this is variable, between 2 and 6 days after fertilisation. When it comes to a transfer of frozen embryos or donor embryos, we will count as the date of fertilisation the day the woman begins taking progesterone since this hormone appears in the body coinciding with the day of ovulation.

To put it plainly, the due date would be 280 days after the first day of the last period only in women with regular 28-day cycles.

For other women, who know the exact date of fertilising intercourse, ovulation, fertilisation, insemination or the day they start taking progesterone, the probable delivery date of 266 days after this day and the corrected date of the “last period” as the 14 days prior to fertilisation shall be considered when counting the traditional 280 days.