Sperm cells reach the oviducts within 30 min of mating, and fertilization can occur within 2–6 hr. Fertilization rates approach 100% in sows, but embryo mortality up to 30%–40% accounts for the usual litter size of 10–16 pigs. Embryos enter the uterus ~48–60 hr after ovulation. Embryos hatch from the zona pellucida and form blastocysts 144 hr after ovulation. Maternal recognition of pregnancy (embryos secreting estradiol) occurs by day 10–14 of gestation, with intrauterine migration and distribution of embryos. Embryo attachment begins by day 13–14, with implantation complete by day 40; a minimum of four embryos must be present at this time for pregnancy to continue. Skeletal mineralization develops by day 35, with fetuses immunocompetent by day 70–75. Fetal deaths that occur after day 35 can result in expulsion or retention of recognizable piglets. Retained dead fetuses in this sterile environment become mummified and are usually expelled at the time of farrowing. The average gestation length is 114 ± 2 days and is somewhat shortened in sows with large litters.
Embryos are at greatest risk of dying during the first 30 days, and efforts should be directed toward avoiding stresses to the sow (eg, overfeeding, heat, handling or moving, immunization) during this critical period. Pregnancies of <16 days are especially sensitive to heat stress. Avoiding exposure to outside animals reduces disease risk. Feed intake should be reduced to the limit feeding level of 4–5 lb (~2 kg) immediately after breeding to avoid embryo loss due to high energy intake. Farrowing less than five piglets is indicative of embryo death after the time of attachment.
To increase colostral antibodies, the gilt or sow should be immunized during the last 6 wk of gestation. An immunization program may include vaccination against Escherichia coli, atrophic rhinitis, and erysipelas, and provision of any other vaccines appropriate for the disease situation on the individual farm.
Several techniques are available for pregnancy determination (see Table: Common Tests for Detection of Pregnancy In Pigs). Pregnancy is most commonly diagnosed by noting that the female does not return to estrus in 18–25 days; this is 75%–85% accurate. Ultrasonography is another popular technique, and three types can be used: pulse echo (A-mode), Doppler, and real-time. Pulse echo or amplitude depth involves emitting ultrasonic waves from a hand-held transducer placed on the skin in the flank area. Reflected waves from a fluid-filled area (ie, developing conceptus or fetus) are picked up by the transducer and converted into either an audible or visual signal. Doppler ultrasonography detects changes in sound frequency (fluid movement) using an audible signal; movements indicative of pregnancy include blood flow in middle uterine or umbilical arteries, fetal heartbeat, and fetal movements. Real-time ultrasonography involves visualization of a 2-dimensional image of scanned tissues directly under the transducer. Ultrasonographic techniques are generally used at 22–75 days to determine pregnancy, with real-time ultrasonography being used as early as 18 days after breeding. Although uncommonly used for this purpose, rectal palpation can be performed to confirm pregnancy at >30 days gestation. The examiner palpates for fremitus, size, and position of the middle (medial) uterine artery in relation to the external iliac artery. The tone and tension of the cervix and weight and contents of the uterus can also be used to help confirm pregnancy. Other techniques such as hormonal assays (eg, estrone glucuronide, progesterone, prostaglandin) and vaginal biopsy can be used but are not economically feasible.