Equine Pregnancy

Question 1

What is the average gestational length of equine pregnancies? How many offspring do they typically have? What is their placentation like?

Answer 1

340 days

monotocous - single offspring per pregnancy

diffuse, epitheliochorial, microcotyledonary, adeciduate –> less invasive = can breed in foal heat without significant waiting period

Question 2

What are 5 important days of equine pregnancy?

Answer 2

1. 5.5/6 = embryo enters uterus
2. 10-15 = embryo in the mobile phase with maternal recognition
3. 16 = embryo fixation - stops moving and resides in a horn (not necessarily where ovulation occurred or where the CL is)
4. 36-40 = formation of endometrial cups (secrete eCG)
5. 120 = fetal-placental unit becomes dependable for hormone secretion and maintenance of pregnancy

Question 3

When does the sperm reach the uterine tubes? Where does fertilization occur?

Answer 3

4-6 hours post insemination

uterine tube - ampillary-isthmic junction

Question 4

When will the oocyte enter the uterus? How does it move? Does it become fixated?

Answer 4

5.5-6 days after ovulation, fertilized oocytes secrete PGE2 and opens up the ampullary-isthmic junction - any unfertilized oocytes remain in uterine tube

migrates between uterine horns with maximal activity at days 11 and 12

yes - day 16

Question 5

What is the embryonic capsule? What does it do?

Answer 5

glycoprotein protective coat formed by the trophoblast layer beneath the zona pellucida

protects embryo and maintains globoid shape –> allows for earlier U/S diagnosis compares to bovines

Question 6

What forms the endometrial cups? When do they regress?

Answer 6

trophoblast cells at the chorionic girdle invade maternal tissues by days 36-38 and secrete eCG around 40 days with peak at 70-90 days

120-150 –> immune response breaks down cups (prolonged eCG effect is seen with persistent endometrial cups pass these days)

Question 7

What 2 effects does eCG have on the ovariwa?

Answer 7

LH-like in horses (FSH-like in other species, used for superovulation)

1. stimulate progesterone secretion from primary CL - maintains primary CL days 35-120
2. stimulate the formation of supplementary CLs - secondary CL formed after ovulation, accessory CL luteinized without ovulation

(multiple CLs originally formed following day 35 to ensure there is enough progesterone to maintain pregnancy)

Question 8

What would happen with eCG levels if abortion occurs after day 35 of pregnancy?

Answer 8

endometrial cup is producing eCG at this point and will not regress until immune reactions within 120-150 days

• will continue maintaining CLs!

Question 9

What is the equine fetoplacental unit?

Answer 9

by around day 120, the fetus and placenta become hormonally active, able to produce progestogens/5a-pregnanes and estrogens (estrone sulphate, equilin, equilenin)

• mare transfers substances to the fetus, which is able to use them to create progestogens and estrogens
• no ovary involved at this point, if for some reason the ovary is lost or dysfunctional, the pregnancy can be maintained

Question 10

What palpation can confirm pregnancy diagnosis in horses?

Answer 10

elongated cervix and increased uterine tone due to increased progesterone + enlarged gravid horn

Question 11

What 2 things can be seen on ultrasound to diagnose pregnancy? When can fetal sex determination be done?

Answer 11

1. embryonic vesicle - can be seen at day 10, but most commonly done at 12-15 days
2. embryonic heartbeat - day 25

• DAY 60 - location of genital tubercle
• DAYS 120-150 - fetal gonads, external genitalia

Question 12

How can a uterine cyst be differentiated from embryonic vesicles?

Answer 12

• scan and map for cysts before breeding
• embryo will be in a highly motile stage days 10-15 after ovulation, but cysts will not
• embryos will grow

Question 13

Embryo, days 21-25:

Answer 13

large yolk sac above small embryo

allantoic cavity is growing and yolk sac is becoming slightly smaller

Question 14

Embryo, days 30-40:

Answer 14

approximate 50/50 split between yolk sac and developing allantoic cavity (Pokeball)

tiny yolk sac with a large allantoic cavity

Question 15

What 3 hormonal tests are used for equine pregnancy diagnosis?

Answer 15

1. progesterone/total progestogens - LOW = not pregnany or luteal insufficiency (going to lose pregnancy); HIGH = from primary CL (<150 days), from fetoplacental units (5a-pregnanes >120 days)
2. eCG/PMSG - secreted by endometrial cups from days 45-100 days
3. estrone sulfate- produced by fetal-placental unit from day 90 (110) to term

Question 16

What is the most common cause of twin pregnancies in horses? What breed does this more commonly happen in?

Answer 16

2 ovulation –> 2 CLs, mostly dizygotic
+/- single embryo transfer - zona pellucida separates and cells emerge

Thoroughbred

Question 17

What are 4 outcomes in equine twin pregnancies?

Answer 17

1. spontaneous reduction - this commonly happens due to their competition for nutrition
2. abortion of both in mid/late gestation
3. dystocia
4. stillborn or weak foals

Question 18

What is key to managing twin equine pregnancies? When is it diagnosed?

Answer 18

early detection

first pregnancy examination before fixation allows for better separation

Question 19

What treatment is recommended if a twin pregnancy is diagnosed on first pregnancy examination? What is done following?

Answer 19

manual reduction or crush with ultrasound probe/hand prior to fixation (<16 days) + NSAIDs + exogenous progestogen (Altrenogest, Regumate)

recheck in 1 week to confirm singleton pregnancy

Question 20

What are 5 options if a twin pregnancy was diagnosed after fixation?

Answer 20

1. oscillation and dislocation of fetus from umbilicus
2. transvaginal U/S-guided fetal heart injection
3. transabdominal fetal heart injection
4. head dislocation
5. induced abortion

these treatments following 16 days are not as rewarding