The Science of Making Babies

Embryo transfer
An embryologist usually calls the morning after your egg retrieval to let you know how many embryos you have and to discuss how many you are thinking you will want to transfer (the final decision will most likely have to be made on the day of transfer, when you will have a better idea of how they’re growing). If you’re going to freeze some, the embryologist also gives you some help deciding how many to leave out to grow a few days before picking the best few to transfer. Some centers suggest letting six to ten embryos, if you have that many, grow for a few days. They then pick the best two or
three to transfer and freeze the rest, sometimes growing the remaining embryos to blastocyst stage before freezing all that survive.
The embryos that are frozen after a few days usually have two to eight cells, and their thaw rate may not be as good as 2PN embryos (embryos frozen the day after egg retrieval).
Two days after retrieval, embryology may call again, this time to tell you how your embryos are growing. One hopes they’ve now reached the two- to fourcell stage.

You may get a third call from embryology the third day after retrieval if doing a three-day transfer. Some centers still do transfers two days after retrieval, and some do most transfers five days after retrieval — at five days, the embryo should have reached the blastocyst stage. If your embryos are now four to eight cells, they’re ready for transfer. In addition to being graded by the number of cells, embryos are also graded by the equality and roundness of the cells and by the amount of fragmentation, or broken pieces, that are in the embryo.

Grading an embryo
At some centers the best-looking embryos on day 3 are graded as an 8A, with 8 being the number of the cells and A through F being the degree of fragmentation. Different centers grade embryos differently, so make sure you understand your particular clinic’s grading system.
Does a great-looking embryo improve your chance of getting pregnant? Most
Embryologists would say a cautious yes, depending on your age, uterine cavity, general health, and many other factors. (An “A” embryo in a 40-yearold woman still has a much lower chance of implanting than, say, a “C” embryo in a 30-year old.)

Occasionally, if you’re doing a three-day transfer, one or more of your embryos may already be morulas, meaning that the cells have pulled together and can no longer be counted. Morulas (from the Latin for “mulberry”) usually have a very high rate of implantation, similar to blastocysts; in fact, they’re the embryo stage right before blastocyst

Looking at Your Uterine Lining
Embryos will implant only if the uterus is ready for implantation. Many centers look via ultrasound at the uterine thickness and also the appearance of the lining, called the pattern, to decide whether your embryos have a good chance of implanting after transfer. Your lining may be described as triple lined, also called tri-laminar. This pattern description is shortened to TL in most centers and describes the lining most clinics like to see before embryo transfer. Fertility centers differ on what’s considered a good thickness, but most prefer to see a thickness of at least 7 millimeters.

Assisted hatching
Assisted hatching (AH) is done the morning of your embryo transfer. The embryologist takes a tiny needle with acid on the end of it and barely touches the shell of the embryo, creating a small hole. This helps the embryo “break through” the zona, or hard shell, and attach to the uterus, an action that must
happen a day or so after the embryo reaches the uterus in nature. Embryos created in the lab are theorized to have harder shells than are seen in natural conception, so the little hole, one hopes, gives them a head start on breaking out and hunkering down where they belong.

• Early Pregnancy Loss/Miscarriage
• Early Pregnancy Loss-Introduction
• Diagnosis and treatment of miscarriage
• Possible causes of recurrent miscarriage
• Ectopic Pregnancy
• Molar Pregnancy

Blastocyst Transfer
In most centers, the percentage of live-birth twins is about 25 percent of total births, and triplets somewhere between 5 and 10 percent. Quadruplets and higher are relatively rare, but are considered a problem at almost all clinics because the complication for both mother and babies is very high, and many
babies die of prematurity or are miscarried.

Out of that concern came the concept of blastocyst transfer, the transfer of a five-day-old embryo. Because only 30 to 50 percent of embryos grow to blastocyst stage (see Figure 16-4), centers felt that only the best embryos were going to be transferred. As it turned out, blastocyst transfer of two embryos results in pregnancy rates in women under 35 that are the same as those achieved by the transfer of three embryos on day 3. However, the risk of triplets was greatly decreased.

Considering complications
Blastocyst transfer isn’t for everyone. Because blastocysts are more complicated to grow than three day embryos, requiring multiple media changes to keep up with their increased nutritional needs, many centers find it too difficult to grow enough blastocysts to get to transfer. So if your center transfers only blastocysts, you may end up with nothing to transfer

The second problem comes from the blastocysts themselves. Because they’re already “hatching” out of their shells at the time of transfer, blastocysts seem unusually likely to split into identical twins. Although on the surface, having identical twins doesn’t seem much different than having fraternal twins, the
fact is that identical twin pregnancies are much more problematic than fraternal ones. Because identical twins share the same placenta and sac, twin-twin transfusion syndrome, in which one twin gets too many nutrients and the other not enough, is more common, as are cord accidents, in which one baby gets tangled in the other’s umbilical cord. The incidence of identical twins is about four to five times higher in blastocyst transfer than in normal conception.