Pregnancy and Parturition

Hi, in this post we’ll discuss the role of progesterone in the maintenance of pregnancy as well as the actions of placental hormones and the maternal recognition of pregnancy. We’ll also take a look at the sequence of physiological and hormonal events leading to parturition and the role of oestrogens during pregnancy and parturition. Finally, I’ll explain the normal foetal presentation for parturition and describe the most common causes of dystocia as well as discuss puerperium.

   

Progesterone

Progesterone is the hormone of pregnancy. It is secreted from the corpus luteum (CL) in early pregnancy and the placenta in later gestation and works to create favourable conditions for the development of the foetus. This includes:

• Negative feedback on GnRH (Gonadotropin Releasing Hormone), LH (Lutenising Hormone) and FSH (Follicle Stimulating Hormone). 
•  Stimulates secretion by uterine endometrial glands. 
•  Suppresses uterine myometrial contractions. 
•  Growth and differentiation of mammary glands.

The fate of the corpus luteum varies between species. In the ewe, mare, woman and cow the CL is not needed in the later half of gestation and the placenta takes over the role of progesterone production. In the sow, goat and rabbit the CL has to be maintained throughout pregnancy as the placenta never secretes enough progesterone.

Maternal Recognition of Pregnancy

 Since the corpus luteum is required during pregnancy, the maternal recognition of pregnancy must occur in order to prevent luteolysis. If the animal is not pregnant, oestrogen binds to the oestrogen receptors in the endometrial cells of the placenta and this causes more oxytocin receptors to be present. Oxytocin binds to the receptors and this promotes PGF2α synthesis which induces luteolysis. However, if the animal is pregnant progesterone from the mother and interferons from the foetus block estradiol receptors which prevents oestrogen from binding. This means that oxytocin receptors aren’t produced and thus oxytocin doesn’t bind. This prevents the synthesis of PGF2α and the CL and pregnancy is maintained as progesterone production continues.   

Placental Hormones

The placenta is composed of foetal (chorion) and maternal components (endometrium). It not only permits the exchange of nutrients such as glucose and amino acids, but toxins such as opioids, barbiturates and heavy metals too. The placenta is known as a transient endocrine organ and it produces hormones that:

·         Stimulate ovarian function

·         Maintain pregnancy

·         Influence foetal growth

·         Stimulate mammary function

·         And assist in parturition.

In mares, the placenta contains transient endometrial cups which begin secreting equine chorionic gonadotropin (eCG) at implantation. eCG is luteotropic and provides the stimulus for the maintenance of the primary CL and thus is responsible for the production of progesterone. High blood levels of eCG cause the development of additional ovarian follicles which luteinize and this results in the development of accessory corpora lutea.

The placenta also secretes lactogen in rats, mice, sheep, cows and humans. Lactogen has growth hormone like effects which promote the growth of the foetus. It also promotes the development of the mammary glands.

In addition, relaxin is also secreted by the placenta in humans, mares, cats, dogs, pigs, monkeys and rabbits. Its synthesis is heightened by PGF2α prior to parturition and causes softening of the connective tissue in the cervix and relaxation of the pelvic ligaments. This prepares the birth canal for the passage of the foetus.

Events Leading to Parturition

 

There are three stages of parturition.

 

Stage 1:

This stage is initiated by the foetus as foetal stress causes ACTH (adrenocorticotropic hormone) to be released from the anterior pituitary. ACTH causes the foetal adrenal glands to secrete cortisol. Cortisol is responsible for the removal of the myometrial progesterone block and increased reproductive tract secretions. During pregnancy progesterone suppresses contractions of the myometrium and this counteracts the effects of oestrogen. Cortisol causes the synthesis of enzymes that convert progesterone to oestrogen. This leads to a rapid drop in progesterone and an increase in oestrogen prior to birth. Cortisol also causes the synthesis of PGF2α which induces luteolysis if the CL is present.

Because the progesterone block has been removed by cortisol, oestrogens cause myometrial contractions to occur and this causes the foetus to put pressure, which is detected by pressure-sensitive neurons, on the cervix. This signals the pituitary to release oxytocin which assists the myometrial contractions that were initiated by oestrogen while PGF2α causes the release of relaxin which softens and dilates the ligaments of the cervix.

Stage 1 is complete once the foetus enters the cervical canal.

Stage 2

This stage involves the expulsion of the foetus and involves a positive feedback loop which ensures the birth is as quick as possible. An increased pressure on the cervix, caused by myometrial contractions and the presence of the foetus, increases oxytocin secretion. In addition, abdominal contractions also help push the foetus out.

Stage 3

In this stage the placenta is normally passed within an hour of birth (in large animals) or with the foetus (in small animals). The continued release of prostaglandins and oxytocin causes uterine contractions and placental detachment.

 Dystocia

 Several conditions may occur which may lead to a difficult birth (dystocia).

 Uterine inertia is the failure to develop and maintain uterine contractions for the normal progression of labour. Primary uterine inertia is when an animal fails to initiate an effective labour pattern and may be genetic, hormonal, mechanical or physical in cause. Secondary uterine inertia is the failure to completely expel all foetuses and is a result of exhaustion. Oxytocin may be administered to an animal with uterine inertia to assist in parturition.

An obstruction may also be the cause of dystocia. The maternal causes of obstruction relate to the abnormalities in the size and shape of the pelvic canal. Foetal causes may be due to the foetus being oversized, foetal anomalies or malpresentation (which is the most common cause of obstruction).

 In animals that give birth to only one offspring at a time, the foetus should be positioned anteriorly (forelimbs first) in a dorso-sacral position. This means that the dorsal aspect of the animal is in line with the sacrum of the mother. With puppies and kittens, both an anterior and posterior presentation is normal.

Peurperium

 Peurperium refers to the time it takes for the uterus to be repaired and for the ovarian activity to resume as normal. It varies significantly between species but has four major events:

1.   Myometrial contractions and the expulsion of lochia (the discharge from the uterus after parturition) 
2.  Endometrial repair 
3.  Resumption of ovarian function
4. Elimination of bacterial contamination in the reproductive tract.

That’s it for this post, if you have any questions, comments or suggestions please let me know :)