Pregnancy in cows and heifers

Conception

Fertilisation of the ovum (oocyte) takes place in the oviduct. The blastocyst arrives in the uterus at around day 5.

microscopic image of oocytes

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Gestation

Gestation in cattle lasts on average for 280 days (279-290d)

  1. Embryonic phase (1- 42d of pregnancy)

    Click here to view images of the embryonic phase

    View images of the embryonic phase

  2. Foetal phase (42d to term)

Proper corpus luteum function is essential for maintenance of pregnancy as this ensures sufficient levels of progesterone.

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Recognition and maintenance of pregnancy

In order to sustain the corpus luteum and maintain pregnancy, the pregnancy has to be recognised. This means that the developing embryo has to produce a specific signal to prevent luteolysis which would otherwise be triggered towards the end of the oestrus cycle.

It has been demonstrated that early bovine and ovine embryos produce and release a specific pregnancy protein – interferon-T (INF- T) (Farin et al., 1989; Mann et al., 1999).
The mechanism of luteolysis inhibition by INF-T is now well established:

  • inhibition of oxytocin receptors on the uterine luminal epithelium
  • induction of a prostaglandin synthesis inhibitor

In cattle, mRNA for interferon-T is first detected in the trophoectoderm, its principle site of production, at approximately 12 days. Maximum levels are reached between days 15 and 16 (Farin et al., 1990). Interferon-T can is first detected in significant amounts in uterine flushings at 14-16 days, this is when embryo elongation begins.(Mann et al., 1998).

Failure of pregnancy recognition

If embryonic development is retarded or if the embryonic growth and the progress of the maternal oestrus cycle are not synchronous (eg due to. delayed ovulation or late insemination) insufficient or delayed INF-T production results. Luteolysis and embryo loss follow.

The main reason for impaired secretion of INF-T by the embryos resulting from fertilisation of oocytes from delayed ovulation is supposedly an ageing process in the oocyte associated with prolonged follicular dominance. It has been argued that due to the prolonged period of follicular dominance and delayed ovulation precocious maturation changes take place in the oocyte, which in turn decrease its fertilisation and developmental capacity. Poor embryonic development in turn is associated with low interferon-T production, failed inhibition of luteolysis and embryo loss (Mann et al., 1996, Man et al., 1998)

For more on failure of pregnancy recognition see Embryonic mortality

Importance of early luteal function in pregnancy recognition

It has been long established that sufficient concentrations of progesterone during early pregnancy have a marked effect on the outcome of insemination. Numerous studies revealed lower concentrations of progesterone in milk (Lamming et al., 1989; Mann et al., 1995) and plasma (Mann et al., 1995, 1996; Buttler et al., 1996; Mann et al., 2001) in cows that fail to maintain pregnancy. Moreover, low progesterone concentrations much earlier in the cycle have been indicated as possible reasons for pregnancy failure.

In the study of Mann et al. (2001) it was demonstrated that the degree of embryo development was closely related to the maternal progesterone environment. Cows with poorly developed embryos on day 16 after the first insemination that produced little or no INF-T exhibited both a delayed increase in progesterone concentrations after ovulation and had a lower luteal phase plateau than cows with well developed embryos.

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blastocyst

Early blastocyst