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Canine sperm can live for 10 to 11 days [22] in the oviducts (fallopian tubes) so if a female is bred 10 days before the oocytes (eggs) can be fertilized, she will appear to have a gestation length of 70 days. If she is bred on the day the oocytes can be fertilized, her gestation length will appear to be 60 days long.
Primordial follicle initiation and development varies by species; in the canine primordial follicle formation occurs 17–54 days post birth. At 17 days, primordial follicles, each of consists of a small oocyte (with no zona pellucida) and a single layer of flattened pre-granulosa cells, can first be observed in the cortical layer of the ovary.
Another factor is due to the shortage of food stocks during winter as the insects are being driven away and as the result, bat hibernate in pregnant condition. [24] In pinnipeds, the purpose of delayed implantation is in order to increase survival chance of the young animals as the mother ensure that the neonates are born at an optimal season. [25]
Female dogs bleed during estrus, which usually lasts from 7–13 days, depending on the size and maturity of the dog. Ovulation occurs 24–48 hours after the luteinizing hormone peak, which occurs around the fourth day of estrus; therefore, this is the best time to begin breeding.
The mean pregnancy length has been estimated to be 283.4 days of gestational age as timed from the first day of the last menstrual period and 280.6 days when retrospectively estimated by obstetric ultrasound measurement of the fetal biparietal diameter (BPD) in the second trimester. [12]
The International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) recommends that pregnant women have routine obstetric ultrasounds between 18 weeks' and 22 weeks' gestational age (the anatomy scan) in order to confirm pregnancy dating, to measure the fetus so that growth abnormalities can be recognized quickly later in pregnancy ...
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Pseudopregnancy can occur when all of the embryos are resorbed after the maternal recognition of pregnancy (days 10–15 postmating) and before fetal bone calcification (days 35–40 of gestation). The sow remains in anoestrus for prolonged periods, often as long as 115 days.