Calculate your expected due date from embryo transfer date and embryo age
Calculating a due date after IVF treatment differs from natural conception because the exact date of fertilisation and embryo transfer is known. This removes much of the guesswork involved in dating a pregnancy from the last menstrual period (LMP). In the UK, the NHS provides IVF treatment through licensed fertility clinics regulated by the Human Fertilisation and Embryology Authority (HFEA), and patients undergoing IVF have precise dates for every stage of the process. The standard pregnancy duration is 280 days (40 weeks) from the first day of the last menstrual period, or 266 days (38 weeks) from conception. For IVF, since the embryo age at transfer is known exactly, the calculation works backwards: due date = transfer date + (266 minus embryo age in days). A day 5 blastocyst transfer means the embryo is already 5 days old at transfer, so 266 - 5 = 261 days are added to the transfer date. NHS-funded IVF cycles in England typically allow one fresh embryo transfer per cycle for women under 40, in line with NICE guidelines (CG156). The choice between day 3 and day 5 (blastocyst) transfer depends on the clinic's assessment and embryo development. Day 5 blastocyst transfers are increasingly preferred in the UK because they allow better embryo selection and have higher implantation rates, though day 3 transfers remain appropriate in certain clinical situations.
To calculate your IVF due date: 1. Select the day, month, and year of your embryo transfer. This is the date the embryo was placed into the uterus, not the date of egg collection or fertilisation. 2. Select the embryo age at transfer. Most UK IVF clinics transfer at day 5 (blastocyst stage), but day 3 and day 6 transfers also occur. Your clinic will have told you the embryo age. Day 6 blastocysts have been cultured one day longer than day 5. 3. Enter the number of embryos transferred. UK guidelines recommend single embryo transfer in most cases, but double embryo transfer may be offered in specific circumstances. This information does not affect the due date but is recorded for reference. 4. View your results. The estimated due date is the primary output. The equivalent LMP date is useful for standard pregnancy tracking -- give this date to your midwife so your pregnancy can be tracked on the conventional 40-week timeline. 5. Note the trimester milestones. Trimester 1 ends at week 12 (the point at which most NHS dating scans occur), and trimester 2 ends at week 27.
The IVF due date formula uses the known embryo age to calculate precisely: Due date = transfer date + (266 - embryo age in days) Day 3 embryo: due date = transfer date + 263 days Day 5 blastocyst: due date = transfer date + 261 days Day 6 blastocyst: due date = transfer date + 260 days Equivalent LMP = transfer date - 14 days - embryo age in days This backdates to what the first day of the last period would have been, allowing standard pregnancy tracking on the 40-week timeline. Gestational age at transfer = 2 weeks + embryo age (because gestational age counts from LMP, which is 14 days before conception) For example, a day 5 blastocyst transferred on 15 January 2026: due date = 15 January + 261 days = 2 October 2026. Equivalent LMP = 15 January - 14 - 5 = 27 December 2025. Gestational age at transfer = 2 weeks 5 days. Trimester dates are calculated from the equivalent LMP: trimester 1 ends at 12 weeks (LMP + 84 days), trimester 2 ends at 27 weeks (LMP + 189 days).
The NHS dating scan at 8-14 weeks may adjust the estimated due date by a few days based on foetal measurements. This is normal and does not indicate a problem. For IVF pregnancies, the scan date and the calculated date are usually very close. If multiple embryos are transferred and more than one implants (twins or more), the pregnancy will be managed differently and delivery is typically earlier. Your NHS fertility clinic and midwife team will provide personalised guidance throughout your pregnancy.