Embryo Transfer: What Happens, and How Many to Transfer

The honest picture, with numbers

Embryo transfer is the quietest step in IVF. A thin catheter is passed through the cervix and the embryo is released into the uterus. The HFEA notes you will not need any kind of anaesthetic unless you have a condition that would make the procedure painful, and the NHS describes it simply as an embryo being put into your womb through a thin tube.

Almost everything that determines the outcome has already happened by this point: how the ovaries responded, how many eggs were collected, how many fertilised, and how the embryos developed. What is still in play on the day is the technique, the number of embryos, and the state of the endometrium — and NICE has something specific to say about each.

What NG257 requires on the day

Ultrasound guidance

Recommendation 1.49.2 says to offer ultrasound-guided embryo transfer during IVF because it improves pregnancy rates. This is the reason you are asked to arrive with a full bladder: an abdominal scan through a full bladder gives the clinician a view of the catheter as it passes. It is uncomfortable and it is doing something.

Endometrial thickness

Recommendation 1.49.3 states that replacement of embryos into a uterine cavity with an endometrium of less than 5 mm thickness is unlikely to result in a pregnancy and is therefore not recommended. If your lining measurement is being discussed, that is the threshold in the UK guideline.

Bed rest afterwards

Recommendation 1.49.4 tells clinics to provide the information that bed rest of more than 20 minutes' duration following embryo transfer does not improve the outcome of IVF treatment. This is one of the few places where guidance directly contradicts the folklore. Lying still for a fortnight is not treatment, and nothing you do with your body in the hours after transfer is what decides this.

How many embryos: the actual rules by age

This is where NG257 is far more granular than most summaries admit. Recommendation 1.49.6 sets a strategy by age and by cycle number:

Three further rules sit on top. Recommendation 1.49.9: where a top-quality blastocyst is available, use single embryo transfer, at any age. Recommendation 1.49.8: do not transfer more than 2 embryos during any one cycle of IVF. Recommendation 1.49.7: if treatment is with donor eggs, base the strategy on the age of the donor, not on yours. And 1.49.10 asks clinicians to discuss the risks of multiple pregnancy whenever double transfer is considered.

The reason for all of this is visible in the national data. The HFEA reports that single embryo transfer accounted for 84% of IVF cycles in 2024, and that the average multiple birth rate from IVF fell to 3.2% that year, among the lowest in the world, while birth rates continued to rise. Transferring fewer embryos did not cost the UK success; it removed a risk.

Day 3 or day 5

Embryos are transferred either at the cleavage stage, selected on day two or three, or at the blastocyst stage on day five. The HFEA states that roughly three quarters of patients in the UK have a blastocyst transfer and a quarter a cleavage stage transfer, and that blastocyst transfers tend to have higher birth rates because the extra days in the laboratory make it easier for the embryologist to identify the embryo most likely to implant.

The trade-off is real, and the HFEA states it plainly: not all embryos left to the blastocyst stage will survive, and in some cases there is nothing left to transfer. There is no way of knowing whether an embryo that stopped on day four would have continued had it been transferred on day three. This is why clinics are more cautious about pushing to blastocyst when there are only one or two embryos, or when a patient is older.

What NICE says not to buy

Recommendation 1.49.1 says not to offer assisted hatching, because it has not been shown to improve pregnancy rates. NG257 also recommends against endometrial scratch and hysteroscopy as pre-treatment, against endometrial receptivity testing including receptivity arrays and endometrial microbiome analysis, and against immunological agents including intralipids, intravenous immunoglobulins and steroids. Separately, the HFEA maintains a rated list of treatment add-ons using a five-colour system, on which several transfer-adjacent extras sit at amber, grey, black or red rather than green.

What to do next

1. Ask how many embryos are being transferred and why, and hold the answer against the age band in recommendation 1.49.6.
2. Ask whether the transfer will be ultrasound-guided. NG257 says it should be.
3. Ask what your endometrial thickness is and what the clinic's threshold for proceeding is.
4. Ask what happens to the embryos you do not transfer. NG257 recommendation 1.49.11 says to offer cryopreservation of any remaining good-quality embryos, and storage carries an annual cost.
5. Check any proposed extra against the HFEA's rated add-ons list and against NG257 before agreeing to pay.
6. Confirm the test date. The NHS says you will be given a pregnancy test and a date to use it, around 16 days after transfer, and that taking it on the given date matters for accuracy.

When to seek help

Mild cramping and a small amount of spotting after a transfer are common and not in themselves a sign of anything. Contact the clinic rather than waiting if you develop increasing abdominal swelling, significant pain, breathlessness, vomiting or a marked drop in how much urine you are passing, because those point towards ovarian hyperstimulation rather than the procedure. The NHS advises going to A and E for difficulty breathing, chest or upper back pain, dehydration with reduced urination, or one-sided low abdominal pain, which can indicate severe hyperstimulation or an ectopic pregnancy.