Lecture 22- Equine Pregnancy and Twinning

What is the primary source of equine chorionic gonadotropin (eCG)?

Endometrial cups

During what days of gestation is eCG present?

Approximately Days 40-120

What is another name for eCG?

Pregnant Mare Serum Gonadotropin (PMSG)

What is the primary action of eCG in the mare?

Acts like LH to stimulate formation of accessory/secondary corpora lutea

What are secondary corpora lutea?

Ovulated corpora lutea formed during pregnancy

What are supplemental corpora lutea?

Luteinized follicles that form without ovulation

What is the purpose of secondary and supplemental corpora lutea?

Provide additional progesterone support during pregnancy

What happens to progesterone concentrations during mid-to-late gestation?

Progesterone remains relatively low

Until what day is the mare CL dependent?

Approximately Day 90-100

When is a mare no longer CL dependent?

After approximately Day 90-100

What estrogen rises after approximately Day 80 and can be used as a pregnancy indicator?

Estrone sulfate

When does estrone sulfate become diagnostically useful for pregnancy detection?

After approximately Day 80

What hormonal change occurs just before foaling?

Progesterone rises

What is the average length of gestation in the mare?

About 340 days (11 months)

What is the normal gestation range in mares?

Approximately 320-380+ days

When does the equine embryo enter the uterus?

Day 5-6

What critical event occurs around Day 17 of pregnancy?

Embryo fixation or prostaglandin release

Why is Day 17 significant in equine pregnancy?

It marks fixation of the embryo within the uterus

When can a fetal heartbeat first be detected ultrasonographically?

Approximately Day 25 (as early as Day 21 with newer ultrasound units)

When do implantation and endometrial cup formation occur?

Day 35-40

When does placental formation begin?

Around Day 35-40

When are endometrial cups fully regressed?

Approximately Day 100-120

Why are endometrial cups important in equine pregnancy?

They produce eCG

How large is the pregnant uterus at 25-30 days?

Comparable to a golf ball or hen's egg

How large is the pregnant uterus at 35-40 days?

Comparable to a tennis ball or baseball

How large is the pregnant uterus at 45-50 days?

Comparable to a softball or grapefruit

What aspect of the uterine horn must be palpated during early pregnancy?

The ventral aspect

Until what day is the fetus generally palpable per rectum?

Day 150

Why is fetal palpation difficult between Days 150-210?

The fetus is positioned over the pelvis

What occurs to fetal position by approximately Day 330?

The fetus moves dorsally

Where are the rear feet located near Day 330?

In the gravid horn

What are the major non-infectious causes of abortion in mares?

Twinning, hydrops, twisted umbilical cord, uterine body pregnancy, trauma, treatments, nutrition, endocrine dysfunction

What is the most common cause of non-infectious abortion in mares?

Twinning

What reproductive pathology commonly causes abortion through extensive fibrosis?

Endometrosis/EED-associated uterine fibrosis

What uterine lesion may contribute to abortion and is commonly associated with EED?

Endometrial cysts

What placental finding should be evaluated when endometrial atrophy is suspected?

Placental pathology

What cervical abnormality can contribute to pregnancy loss?

Cervical lesions

How can poor perineal conformation affect pregnancy?

Predisposes to reproductive failure

Why can inflammatory diseases result in abortion?

PGF2α release may induce pregnancy loss

How can surgery contribute to abortion?

Resulting maternal hypoxia

What non-pathologic factor is listed as contributing to pregnancy loss?

Stress

What are monozygotic twins?

Twins arising from one ovulation and one fertilized ovum that splits

What is another term for monozygotic twins?

Identical twins

What genetic relationship exists between monozygotic twins?

Identical genomes

How are monozygotic twins thought to arise?

Splitting of a fertilized ovum during blastocyst hatching

What role can the zona pellucida play in identical twinning?

Pinching effect during blastocyst hatching

What are dizygotic twins?

Twins arising from two separate ova

What is another term for dizygotic twins?

Fraternal twins

Do dizygotic twins have identical genomes?

No

Can two eggs with one ovulation produce twins?

Yes, rare dizygotic twins

What percentage of equine twins are dizygotic?

Greater than 95%

What is the primary cause of equine dizygotic twinning?

Asynchronous ovulations

What defines synchronous ovulations?

Ovulations occurring within 24 hours of each other

What defines asynchronous ovulations?

Ovulations occurring more than 24 hours apart

Why are monozygotic twins uncommon in horses?

The embryonic capsule may prevent embryo splitting

Which breeds have the highest incidence of twinning?

Thoroughbreds and Draft horses

Which breeds have intermediate twinning rates?

Quarter Horses and Arabians

Which breeds have the lowest incidence of twinning?

Ponies

Which mares have a higher incidence of twinning based on age?

Mares 6-10 years old

How does foaling status affect twinning risk?

Increased incidence within 80 days postpartum

How do ovulatory drugs affect twinning risk?

Increase twinning incidence

What percentage of twin pregnancies result in birth of a single foal?

60%

What percentage of twin pregnancies result in loss of both pregnancies?

31%

What percentage of twin pregnancies carry both fetuses to term?

9%

Of twin pregnancies carried to term, what percentage result in both foals dead?

65%

Of twin pregnancies carried to term, what percentage result in one live foal?

21%

Of twin pregnancies carried to term, what percentage result in both live foals?

14%

Why is twin pregnancy considered undesirable in mares?

Placental insufficiency is common

What fetal survival problem is associated with equine twin gestations?

Rare for both fetuses to survive to birth

How can twinning affect parturition?

Increased likelihood of foaling assistance

How are surviving twin foals typically affected?

Tend to be weaker

What postpartum complication is more common in mares carrying twins?

Retained placenta

What is the preferred method for diagnosing twins?

Transrectal ultrasonography

When should mares be examined for twins?

Day 14-16 post-ovulation

Why is Day 14-16 considered the optimal examination period?

Before embryo fixation and ideal for reduction

What is a unilateral (unicornual) twin pregnancy?

Both embryos located in the same uterine horn

What is a bilateral (bicornual) twin pregnancy?

Embryos located in separate horns

When can transabdominal ultrasound be used for twin diagnosis?

After approximately 80 days

Why is hormone assessment unreliable for diagnosing twins?

Increased progesterone from multiple CLs is not diagnostic

What natural mechanism may reduce twin pregnancies after fixation?

Natural twin reduction

When is natural reduction most likely to occur?

Around cessation of embryonic mobility

How does asynchronous ovulation affect natural reduction?

Makes reduction less efficient

How does bilateral fixation affect natural reduction?

Makes reduction much less likely

What contributes to natural reduction of one twin?

Nutrient deprivation

What characterizes unilateral fixation of twins?

Early membrane-to-membrane contact

How does unilateral fixation affect nutrient exchange?

Reduces nutrient exchange from the endometrium

What percentage of twin pregnancies are unilateral?

Approximately 70%

What percentage of unilateral twins undergo natural reduction?

Approximately 85%

What characterizes bilateral fixation?

Late membrane-to-membrane contact

How long do both embryos often survive in bilateral fixation?

Beyond Day 40

What percentage of twin pregnancies are bilateral?

Approximately 30%

How common is natural reduction in bilateral twins?

Almost non-existent

What breeding management recommendation applies to all mares regarding twins?

Examine for twins regardless of double ovulation status

What is the preferred management for twins at Days 14-16?

Manual reduction/crushing

How should unilateral twins be managed from Days 17-25?

Consider waiting for natural reduction

How should bilateral twins be managed from Days 17-25?

Manual reduction

What is the principle behind embryo crushing?

Reduce one conceptus while preserving the other

Which embryo should be selected for reduction?

The smaller embryo or the most accessible embryo

What is the easiest twin reduction technique?

Manual or ultrasound-assisted crushing

When is crushing most effective?

Before fixation (before Day 16)