During IVF, the embryo is placed inside the uterus, specifically in the upper portion of the uterine cavity near the top (called the fundus). The catheter tip is typically positioned 5 to 15 millimeters from the fundus, and the embryo is released into that space so it can settle against the uterine lining and implant naturally from there.
Exactly Where the Catheter Goes
The uterus is a pear-shaped organ, and the fundus is the broad, rounded top. During embryo transfer, a thin, flexible catheter is threaded through the cervix and into the uterine cavity. The goal is to deposit the embryo close to the fundus but not touching it, usually 5 to 15 mm away. Placing it too close risks disturbing the lining; placing it too far away gives the embryo a longer journey to reach the ideal implantation zone.
Simulation studies show that a medium distance of about 10 mm from the fundus, combined with a moderate injection speed, gives the highest rate of embryo deposition in the fundal area. This matters because the upper portion of the uterus has the richest blood supply and thickest lining, making it the most favorable spot for an embryo to attach and grow.
How the Transfer Is Done
Most clinics use abdominal ultrasound to guide the catheter in real time. A full bladder helps create a clearer image of the uterus on the screen, which is why you’ll be asked to drink water beforehand. The doctor watches the catheter tip on the ultrasound as it advances through the cervix, then releases the embryo at the target location. You can often see a small bright flash on the screen, which is the fluid containing the embryo leaving the catheter.
The American Society for Reproductive Medicine recommends ultrasound guidance based on strong evidence from multiple randomized trials showing it improves both clinical pregnancy rates and live birth rates compared to a blind or “touch” technique. The entire transfer takes only a few minutes and is typically painless, similar to a Pap smear in terms of discomfort.
What Happens After the Embryo Is Released
Once placed, the embryo doesn’t necessarily stay exactly where it lands. Research tracking the ultrasound flash marker after transfer shows that embryos commonly migrate within the uterine cavity over the following hour. Most drift toward the fundus or remain in their original position. Gentle, wave-like contractions of the uterine wall play a major role in this movement, essentially guiding the embryo toward its final resting place.
This post-transfer migration explains why the exact millimeter of catheter placement matters less than getting the embryo into the right general zone. It also means that factors like uterine contractility and endometrial thickness can influence the outcome as much as the precision of placement itself. Women with a thinner endometrium appear more likely to have embryos drift toward the fallopian tubes, slightly raising the risk of ectopic pregnancy. A thicker lining may push embryos in the opposite direction, toward the cervix.
The overall ectopic pregnancy rate after IVF is about 2%, meaning the embryo implants outside the uterus (usually in a fallopian tube) in roughly 1 out of 50 pregnancies. One study found that up to 38% of the transfer fluid can back up into the tubes, though in most cases the embryo still finds its way to the uterine lining.
How the Embryo Attaches to the Lining
Implantation itself happens in three stages. First comes apposition, where the embryo loosely rests against the uterine lining. Next is adhesion, where the outer cells of the embryo form a firmer bond with the surface of the lining. Finally, the embryo invades, burrowing into the deeper layers of the endometrium and connecting to the mother’s blood supply. This entire process takes several days and is identical whether pregnancy happens naturally or through IVF.
For this to succeed, the lining needs to be in a receptive state. Clinical pregnancy rates drop when the endometrial thickness is below 7 mm, and live birth rates tend to peak when the lining measures 10 to 12 mm in fresh transfer cycles. In frozen embryo transfers, outcomes improve once the lining reaches 7 to 10 mm. Your clinic will measure this on ultrasound before scheduling the transfer.
The Window of Implantation
The uterine lining is only receptive to an embryo for a limited stretch of time, often called the window of implantation. In natural cycles, this opens roughly 8 to 10 days after ovulation and lasts about three days. In medicated frozen embryo transfer cycles, the window is timed using progesterone: for day-3 (cleavage-stage) embryos, the receptive period falls between days 2 and 5 of progesterone supplementation, while for day-5 blastocysts, it spans days 3 through 7.
Pregnancy rates within these windows were consistent across the different timing subgroups, which suggests there is some flexibility. Still, clinics time the transfer carefully to land within this window. If there’s concern that your personal window might be shifted earlier or later than average, a specialized biopsy test can help your clinic adjust the timing.
Activity After the Transfer
You may have heard that bed rest improves your chances after embryo transfer. It doesn’t. A systematic review and meta-analysis found that live birth rates were the same whether patients rested after the procedure or got up and resumed light activity right away. Some evidence even suggests that prolonged bed rest could be slightly harmful. Current guidance supports returning to normal, low-impact activity the same day. The embryo won’t “fall out” from walking, climbing stairs, or going back to work.