News Animal Health
Developmental milestones during pig gestation

Fetal development occurs between attachment of fertilized embryos to the uterus, to the days right before parturition (birth). It is at this time, gestation, that piglets form and grow inside of the sow. Generally, the length of time a sow is pregnant, or gestating, is about 115 days. During this time the piglets go through different phases of development, which we will separate into five major phases. This article will cover the first three phases; next issue will cover the final three.

Day 0-15
The first sign of heat is considered Day 0 of the estrous cycle. Ovulation occurs within the first 24 to 48 hours after the start of heat. Fertilized embryos will elongate and migrate to find space in the uterus. During this time, sow fighting and trauma typically have little impact on embryo survival, because the embryos have yet to form attachments to the uterine wall. Any loss in embryos at this time is typically influenced by quality of the oocyte before ovulation, quality of the sperm, and/or disruptions in hormonal signalling before fertilization. Embryo attachment to the uterus occurs around Day 12-15. In swine, four or more viable embryos must establish an attachment to the uterine wall in order for the pregnancy to be viable and continue past this point. During the attachment phase, embryos begin aligning along the uterine horn.

If enough viable embryos attach, recognition of the pregnancy will occur by Day 11-12. Embryos that fail to attach to the uterine wall will not survive the pregnancy. If an insufficient number of embryos (less than four) attach, the pregnancy won’t be recognized, and the sow will return to heat by Day 21. If the embryos successfully attach to the uterine wall, they will begin to form in their own placenta, as each piglet has their own placenta. Pigs have an epitheliochorial placenta, which means that the placenta does not invade the uterine tissue like other types of placenta. This type of placenta sits against uterine wall, and forms a grooved attachment, often compared to velcro. Each piglet having its own placenta isolates each fetus from the rest of the litter and prevents the loss of one fetus from affecting the survival of the other fetal pigs. However, this also means each placenta needs adequate attachment to the uterus, and a lack of space can impact placental development for any pig. This means that the larger the litter, the less space for placental attachment, which can lead to smaller piglets.

Day 13-30
In this time range, the initial placental expansion begins. Around day 30, sows can be checked for pregnancy via ultrasound. When using an ultrasound machine, fluid filled sacs indicate that a litter is developing properly. In group gestation housing systems, if sows were not housed together immediately after breeding, it is best to wait until after confirmed pregnancy to group sows together. By this time, placental attachment is considered sufficient to survive fighting that might occur between sows. 

Day 30-77
During this time noticeable organ development begins. Bones begin calcifying at Day 35-45. Sometimes, for a variety of reasons, individual fetal pigs will stop developing and die in the uterus. Fetal pigs that die during this time of gestation can lead to the presence of mummies at farrowing. A mummy is a fetus that died after calcification of bone has occurred, and therefore, cannot be reabsorbed. Rather, it decomposes and mummifies in its placenta. For a pig producer, a litter that has many mummified fetuses found at farrowing can be a sign of trauma to the sow or developing offspring during the earlier stages of gestation. Trauma can include rough handling, poor nutrition, environmental stressors, or disease stress.