Evidence-Based ET: Where does the transferred embryo have to be placed in order to maximize pregnancy rate?

Categories: Evidence-Based ET
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Published on: March 13, 2014

Note: This is the fourth in the series of topics suggested by the AETA Board of Directors. Should any readers want to suggest future topics, please email them to me.

John F. Hasler


Where does the transferred embryo have to be placed in order to maximize pregnancy rate?

Well, we know where a bovine embryo belongs when it first travels through the UT junction!  Initially, it will be in the tip of the uterine horn, and if the embryo resulted from fertilization of the animal’s own ovulated oocyte, then the embryo will be in the horn on the same side (ipsilateral) as the CL. However, that is not necessarily where the embryo will end up and implant.  Embryo migration within the uterus is well-known in litter producing domestic animals, such as swine, but it also has been shown to occur in single ovulating cattle at a frequency of 2 to 3% (for review see McMillan and Peterson, 1999), although Scanlon (1972) reported no trans-uterine migration in more than 600 single ovulating cows. He did, however, note that a series of previous published reports indicated a migration percentage of 0.3 to 1.6%. Migration has not been reported in cattle following the non-surgical transfer of a single embryo transferred to the horn either to the ipsilateral or contralateral to the ovary with the CL, although the sample sizes were small.

Discounting the importance of embryo migration as an issue in cattle, it is well known that the presence of an embryo in the horn on the side ipsilateral to the CL is important to maximize the odds of achieving pregnancy. In the absence of an embryo, there is no recognition of pregnancy and the female will return to estrus in a few days. Starting about day 13, a protein named interferon-tau (IFN-tau) is secreted by the trophoblast cells of blastocysts (Hansen, et al., 1999) (Thomas [Tod] Hansen is currently the director of the Animal Reproduction and Biotechnology lab at CSU) and, through a complex series of chemical events, decreases the release of PGF2 alpha (PG) from the uterine endometrium. In the absence of IFN-tau, PGF2 alpha is normally released about day 16, the CL is lysed and the animal returns to estrus in a few days. If an embryo is transferred to the contralateral horn, the IFN-tau signal may not sufficiently reduce the release of PG to prevent a return to estrus. Several studies, including Sreenan, et al. (1975) and Tervit, et al. (1976) have documented the reduction in pregnancy rate resulting from the transfer of an embryo to the ipsilateral horn , versus the contralateral horn (69 v 31% and 54 v 39%, respectively). Clearly, then, it is not a disaster to transfer an embryo into the ‘wrong’ horn, but it will usually seriously reduce the chance of establishing a pregnancy. If pregnancy can occur even when an embryo is present in the contralateral horn, then how much difference does it make where it is transferred within the length of the ipsilateral horn?

Increasing the chances of pregnancy involving transfer into the ipsilateral horn was attempted, even before the role of INF-tau was understood, by transferring trophoblastic vesicles along with the embryo. Heyman, et al. (1987) prepared the vesicles by culturing pieces of Day-14 bovine embryos for 24 hours. The vesicles, which averaged 1.2 mm in diameter, were then frozen in liquid Nuntil which time they were thawed and two vesicles were transferred along with each frozen-thawed blastocyst. The resulting pregnancy rates were 73 and 43% for the experimental versus control transfers on Day-45 and 57 versus 40% on Day-90, respectively. Clearly, pregnancy rates are enhanced if the embryo is transferred ipsilaterally, which is normally achieved, and if additional trophoblastic ‘signaling’ is provided. However, most of us do not have access to a source of additional trophoblastic cells, either in the form of large fragments or vesicles. So, once again the question is “where do we put the embryo?”

Bill Beal, working with Randall Hinshaw, published an abstract on this subject (Beal, et al. 1998). Thawed DT embryos were transferred either “deep” (more than 2/3 of the distance from the external uterine bifurcation toward the oviductal end) or “shallow” (adjacent to the external uterine bifurcation). Resulting pregnancy rates were in favor of deep transfers, regardless of embryo quality, versus shallow transfers, 66% versus 30%, respectively.  I am not aware of any other published studies dealing with this subject.  However, Richard Steel shared a very large data set on this subject with me.  We have never published these data, but I have used them in several presentations at ET/cattle meetings. Analyzing more than 6,000 transfers, we found that the pregnancy rate varied when the embryo was transferred in the upper, middle or lower third of the ipsilateral uterine horn. The pregnancy rates, for all embryo grades combined, were 69.6, 68.8 and 59.6% for the three transfer depths, with the rate significantly higher for the upper and middle thirds compared to the lower third. Interestingly, as shown in Table 1, there was no influence of depth of transfer when the embryos were rated as Grade 1. However, for grade 2 or 3 embryos the pregnancy rate was successively lower for embryos transferred into the middle or lower third of the uterine horn. These are provocative data, and I am going to ask Rick Steel if he has additional data to add to this set and then co-publish the results. Dr. Steel has recently retired from active ET and did a very fine job of recording many factors involved in his ET practice.

Table 1. Effect of embryo grade and depth on transfer on pregnancy rate in bovine ET recipients (Steel, unpublished)

Embryo Grade




Transfer depth No. (% pregnant) No. (% pregnant) No. (& pregnant)
Lower  1/3

  75      (73)

 41    (49)

23      (26)

Middle 1/3

291     (73)

151    (59)

55      (35)

Upper  1/3

493     (74)

209    (66)

91      (41)


  859    (73.5)

   401   (61.3)

169   (36.7)


The take home message from all of the above is that one should try to deposit the embryo as far up the horn as possible, especially in the case of lower quality in vivo-derived fresh embryos and probably also in vitro-derived embryos, biopsied and frozen-thawed embryos. However, it may be more important to complete the transfer with a minimum of struggling to get high in the horn, thus reducing the chance of bleeding and endometrial damage.



Beal, W.E., Hinshaw, R.H., Whitman, S.S. 1998. Evaluating embryo freezing method and the site of embryo deposition on pregnancy rate in bovine embryo transfer. Theriogenology 49:24.

Hansen, T.R., Austin, K.J., Perry, D.J., Pru, J.K., Teizeira, M.J., Johnson, G.A. 1999. Mechanism of action of interferon-tau in the uterus during early pregnancy. J. Reprod Fert Suppl. 43:329-339.

Heyman, Y., Chesne, P., Chupin, D., Menezo, Y. 1987. Improvement of survival rate of frozen cattle blastocysts after transfer with trophoblastic vesicles. Theriogenology 27:477-485.

McMillan, W.H. Peterson, A.J. 1999. Transuterine embryo migration in recipient cattle. Theriogenology 51:1577-1586.

Scanlon, P.F. 1972 Frequency of transuterine migration of embryos in ewes and cows. J. Anim. Sci. 34:791-794.

Sreenan, J.M., Beehan, D., Mulvehill, P. 1975. Egg transfer in the cow: factor affecting pregnancy and twinning rates following bilateral transfers. J. Reprod. Fert. 44:77-85.

Tervit, H.R. Havik, P.G., Smith, J.F. 1976. Egg transfer in cattle: Pregnancy rate following transfer to the uterine horn ipsilateral or contralateral to the functional corpus luteum. Theriogenology 7:3-10.


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