Gestation and Parturition

Feeding during Pregnancy

The nutrient requirements of a pregnant female throughout gestation changes in proportion to daily gain of the fetus. I will use the pregnant ewe to illustrate this. The feeding practices described are specific to managing animals in New Zealand and may not be suitable for every flock.

The graph on the right was plotted by measuring the weights of fetuses from Romney ewes at varying stages of gestation. The observed pattern of fetal growth is exponential, which is expected since the total growth curve of an animal is generally sigmoidal. During the first trimester (~50 days), the fetus is less than 200 grams. This hardly causes stress to the ewe since the nutrient requirement is so small. The ewe can generally be fed at 0.8% maintenance if she is in good condition at breeding.

During mid-pregnancy, the fetus begins to show significant growth. This increases the nutrient requirement, and the ewe should be fed at or above maintenance so that she does not lose too much condition.

In the third trimester, exponential growth of the fetus puts a high energy demand on the ewe. During this time, if the ewe is not gaining weight it means that she is actually losing condition while the fetus grows.

Days % Birth Weight

0-75 (1/2 pregnancy) <10

0-100 (2/3 pregnancy) ~25

last 30 >50

Species Gestation Length (days)

Pig 114

Horse 330 (11 mo.)

Cow 283

Sheep/goat 152

Dog/cat 63

The gestation period is the amount of time it takes for a fetus to develop within the mother up until the time of birth. Fetal development may be influenced by a number of factors including the type of environment the mother is exposed to, body condition of the mother, stress level, and nutrient availability. "Fetal programming" is now being explored in depth, a theory that during critical periods in early fetal development, there are changes in the body structure and physiology that occur due to environmental stimuli and persist throughout the lifetime of that animal.

Parturition

Blood vessels of the placenta all gather into the umbilical cord. In cows, there are about 80 caruncles with densely arranged blood vessels. Any hormonal change in the mother or fetus will be detected by the other. This is especially important come birthing time, when the calf's rising levels of fetal adrenocorticotropic hormone initiate parturition. To prepare for expulsion of the fetus, the cow goes through a series of hormonal changes.

Fetal Stress

Increase Fetal ACTH

Fetal Cortisol

Placental P4

E2

Relaxin

PGF2alpha

ENZYMES

Secretions by female tract

Myometrial contractions increase

Luteolysis kills CL, other P4 producer

P4 block is removed

Pelvic ligament stretching

Dilate cervix

Increase pressure

Increase cervical stimulation

increase oxytocin

Max. Pressure (parturition)

Lubrication

Dexamethasone is like cortisol, but is synthetic. It has a longer half life and may be used to induce parturition, although there are many implications for future health of the calf.

When fetal ACTH is prevented from getting to the adrenal cortex of the cow, the result is delayed parturition.