Animal Gestation Periods Quiz

Physiology of Pregnancy

• Presented by Dr. Mohamed Wagdy Rezk Mansour
• Lecturer of Theriogenology
• Theriogenology department, Faculty of veterinary medicine, University of Sadat City
• [email protected]

Gestation Period/Pregnancy Duration

• Period of time from fertilization to parturition or from conception till birth

Factors Affecting Gestation Period Length

• I. Hereditary factors:
  • Species: Monotous animals (cow and mare) have longer gestation periods than polytocus animals (sheep and goats). Beef cattle have longer gestation periods than dairy cows.
  • Breed: Differences in gestation length exist between breeds.
  • Fetal genotype: Hybrids (e.g., horse x donkey) have gestation periods closer to the paternal species than maternal species. Mare x stallion = foal: (320-360) days. Mare x donkey = mule: (360-380) days.
• II. Environmental factors:
  • Feeding: Well-fed animals have shorter gestation periods (by 3 days) compared to those that are ill-fed.
  • Season: Animals born in summer or autumn have shorter gestation periods than those born in winter or spring.
• III. Fetal factors:
  • Litter size: Inverse relationship between gestation period and litter size in polytocus animals. Twin pregnancies (in cows and buffaloes) have shorter gestation periods than single-fetus pregnancies.
  • Number of fetuses: A higher number of fetuses is associated with a shorter gestation period.
  • Sex of fetus: Male fetuses have longer gestation periods than female fetuses.
• IV. Maternal factors:
  • Number of previous parturitions: Primiparous (first-time mothers) animals have shorter gestation periods than pluriparous (multiple-time mothers) animals.

Stages of Gestation Period

• 1. Germinal period (Ovum stage):
  • Starts from fertilization and ends with implantation of the blastocyst in the endometrium.
  • Fertilized ovum takes 3-5 days to reach the uterus (morula stage/ 16-32 cell stage).
  • By 10-12 days, the blastocyst begins to attach to the endometrium.
• 2. Embryonic stage:
  • Begins after implantation, during which the embryo and placenta form (organogenesis).
  • Body differentiation occurs, heart prominence appears by the end of the fourth week, and fore and hind limbs by the end of the fifth week. Fetal membranes form.
  • Timeframes (approximate): Cows (13th – 45th days), Buffaloes (13th-50th days), Mares (29th-55th days), Sheep and goats (10th-35th days).
• 3. Fetal stage: Begins with organogenesis and ends with parturition. The fetus grows in size and different organs develop.

Mechanism of Implantation

• Continuous growth of blastocyst leads to fragmentation of zona pellucida. The trophoblast cells forming finger-like processes (chorionic villi) invade the endometrial mucosa.
• Specific pregnancy lengths vary: Bitches – 7 days; Ewes (sheep) – 10 days; Cows and buffaloes – 12-13 days; Sows (pigs) – 14 days. Mares – 29 days.

In fallopian tube

• Fertilized ovum remains in fallopian tube for 3-5 days
• Nourishment/feeding on yolk sac and nutrient material secreted. Mitotic divided into several cells (morula stage/ 32 cells)
• Migrate to uterus by contraction of oviduct and Ciliary movement of lining epithelium.

Yolk sac

• Nourishes the ovum in early gestation. (absorbing uterine milk from endometrium).
• Atrophied and included within the umbilical cord by the end of early gestation.

Formation of Fetal Membranes

• Fetal membranes surround developing embryo and includes amnion, chorion, and allantois.
  • The amnion is a double-walled sac, surrounds the embryo, and is filled with fluid.
  • The allantois acts as a waste reservoir and connects the developing fetus to the maternal circulation through the umbilical cord.

Amniotic Plaques

• Small (0.5-1.5 cm) white elevated plaques on the inner surface of the amnion, containing calcium deposits.
• Develop in the third month, keratinizing by the 7th month.

Function of the Amnion

• Protective cushion against external shock.
• Prevents adhesions between embryo and surrounding structures.
• Acts as a wedge during parturition, dilating the cervix.

Fetal and Placenta (Amniotic Surface Exposed)

• Umbilical and amnion are covered by amniotic plaques.

Allantois

• Completely formed 24-28 days after conception.
• Arises as an out-pocketing of the hindgut.
• Connected to the fetal bladder via the urachus, which runs through the umbilical cord.
• Forms calculi (hippomanes), dark fleshy masses of cellular debris, fat, and degenerated blood vessels (in cow, sow, ewe, bitch, and mare).

In Ruminants

• Allantois does not completely surround the amnion due to adhesion between chorion and allantois in the dorsal surface. Forms amniochorion membrane

Chorion

• Outer fetal membrane.
• Directly in contact with the uterine endometrium.
• A bare area (cervical star) exists in mares on the chorion facing the internal os.

Fetal Fluids

• Transudated from the fetal membranes (amnion and allantoic).
• Transudated from fetal/maternal circulation.
• Secreted from fetal skin and nasopharyngeal mucosa.
• Fetal urine contains carbohydrates, fat, minerals, NH3, creatinine, uric acid, calcium, phosphorus, sugar, and albumin

Function of the Placenta

• Respiratory organ, carrying O2 to fetus
• Nutritional organ
• Selectively absorbs elements (calcium, phosphorus, iron, and vitamins).
• Stores glycogen and fats.
• Barrier against infectious materials like brucella.
• Conveys antibiotics/antibodies.
• Endocrine organ, secreting hormones (estrogen, progesterone, relaxin, and pituitary-like gonadotropin, placental lactogen).

Hormonal Control of Pregnancy

• Pregnancy is regulated by progesterone.
• Progesterone is essential for endometrial gland growth to produce uterine milk to nourish embryo/fetus and reduce uterine motility.
• Sources of progesterone may vary by species.
  • In cattle and sheep, the corpus luteum (CL) produces progesterone during the early stages, with the placenta taking over during the later stages.

Changes in Uterine Contents During Pregnancy

• Uterine wall thins, and uterine horns become asymmetrical.
• Foetal fluids accumulate, placentomes develop, and foetal membranes form.
• Fetal movement and size lead to changes in uterine position, with gradual changes in pelvic cavity/abdominal cavity from the 3rd-7th month.

Relative Growth of Different Components of Pregnant Uterus

• First half of pregnancy: Foetal membranes grow rapidly, while the foetus grows slowly and there's considerable accumulation of foetal fluid.
• Second half of pregnancy: The foetus grows rapidly in size and weight, with relatively static growth in foetal membranes and fluids.

Classification of the Placenta

• Categorized by layers separating maternal and foetal blood across the placenta. Placenta is classified as simple diffuse, cotyledonary, zonary, or discoidal.

Histological Classification of Placenta

• Six layers: Endothelium of blood vessel of both foetus and mother, Connective tissue, Trophoblast, Endometrium. 
  • Types:  Epitheliochorial, Syndesmochorial, Endotheliochorial, and hemochorial.

Anatomical Classification of Placenta

• Simple diffuse – Whole surface of chorion with finger-like villi embedded in endometrial crypts, separated easily (mare, she-camel, sow, pig).
• Cotyledonary – Caruncles in endometrium lie in apposition with cotyledons, forming placentomes. Placentome are arranged in 4 rows (2 dorsal and 2 ventral): Convex in cows/buffaloes, cup shaped in ewes and goats.
• Zonary – Middle part of the allanto-chorion covered with villi, and interlocked in pockets (dog/cat/bitch).
• Discoidal – Maternal/foetal attachment in a single or multiple disc like areas(mice, guinea pig, woman/primates).

Function of the Placenta

• Fetal respiration, carrying O2 to fetus
• Nutrition
• Selective absorption of vitamins
• Storing glycogen and fats.
• Barrier against infectious agents (brucella)
• Antibiotic and antibodies movement.
• Secretion of hormones (estrogen, progesterone, relaxin, placental lactogen)
• Fetal membrane aids in pregnancy diagnosis
• Production of fructose

Maturation of Fetus

• Whole body covered with hair, except inner thigh.
• Signs of maturity: Weight, Length (CR/CVR), Hair distribution/color/length, Tooth eruption.

Changes during Fetal Stage

• Growth changes: Fetal length increase. Cow/calf: 1cm/week until 11th then 2cm/week afterwards.
• Maturation changes: Hair development around the full body. Fetal weight increases 

The Umbilical Cord

• Connects fetus and placenta.    
• Contains: Two umbilical arteries, two umbilical veins, urachus (vestige of yolk sac), and Wharton's jelly.
• Length varies between species (e.g., buffalo calves - 50 cm, cow calves -30-40cm, foals - 45-90cm, lambs and kids -20cm, puppies/kittens - 8-10 cm).

Importance of Judging Fetus Maturity

• Pay attention to immature animals’ diet and care.
• Crucial management in cases of premature birth or venereal diseases.
• Unripe animals are characterized by small size, unusual hair distribution/color/length, irregular teeth, weakness, and/or decreased activity.

Symptoms of Unripeness in Newly Born Animals

• Small size, irregular hair distribution/color/length, irregular teeth, weak, inactive, can potentially die shortly after.

Test your knowledge on the average gestation periods of various animals, including cows, donkeys, and elephants. This quiz covers factors influencing gestation length, stages of gestation, and specific terminology associated with embryonic development. Perfect for students studying animal biology or veterinary sciences.