Canine Fetal Development

Source: PetWave, Updated on July 16, 2015
Pregnancy

Puppies develop from a fertilized egg to a viable fetus over a very short period of time - about nine weeks (58 – 63 days on average). This period is called “gestation.” Gestation ends when parturition (birth) occurs. Anyone breeding a litter should become familiar with the stages of fetal development so that they can take all steps necessary to attend to the needs of their pregnant dog and her future offspring. Here’s a snapshot of what goes on inside a dog once she has been bred successfully:

Average Day After Breeding

Fetal Developmental Stage

2

Ovulation

5

Fertilization of egg by sperm (now called an ovum); chemical reaction causes outer layer of ovum to harden and form a protective barrier, making it impervious to other sperm

6

Ovum divides (cleaves); becomes a 2-celled embryo

7

2-celled embryo moves to oviduct (between ovary and uterine horn); cells divide

8

4-celled embryo forms; female going out of heat

9

8-celled embryo becomes 16-celled embryo; cells divide

10

16-32-celled embryo (now called a morula); cells divide

11

32-64-celled morula enters uterine horn; fluid accumulates inside morula; morula develops a defined fluid-filled inner cavity (now called a blastocyst)

12-18

Blastocyst floats free in uterus; multiple blastocysts space out evenly along uterine horns; grow from ~¼ mm to ~2 mm in diameter; embryos resistant to insult from chemicals or radiation

19

Blastocysts implant; uterine lining forms band-like structure around each embryo that becomes the placenta

20

Body of embryo starts to form; linear thickening of cells (the “primitive streak”)

21

Primitive body parts start to form (called “somites”)

22

10 or more somites formed; primitive brain and spinal cord starting to develop; head area of embryo bends forward

23-25

Limb buds form; embryo grows from ~5 mm to ~10 mm in length

26

Limb buds become paddle-like; uterine lining swells at each implantation site as embryos grow; skilled practitioner may be able to palpate

27

Pigmentation develops in retina of eyes; body taking form; face starting to take shape

28

Facial features become more defined; nipples develop; embryo now ~14-15 mm long

29

Toes take shape; whisker buds become visible; umbilical cord forms; embryo ~18 mm long

30-32

Details of face and legs become recognizably dog-like; embryo 24 mm long (now called a “fetus”); organogenesis ends; beginning of mammary development

33

Male and female sexual characteristics can be differentiated; fetus ~24 mm long

34

Toes and toenails differentiate; eyelids close; fetus ~30 mm long

35-40

External and internal body parts grow and differentiate; skin pigmentation visible; fetus ~45 mm long

40-42

Beginning of ossification (formation of bone)

41-63

Continued fetal growth and differentiation

55

Fetal movement detectable with abdominal palpation

Birth of puppies is the end result of a complex chain of events known as “whelping.” “Parturition” is the medical term for the act or process of giving birth.

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