The Gravid Uterus

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Uterine changes

• Changes in size, shape & position 

• Development of the decidua (lining of the uterus in pregnancy)

• Growth & development of the muscle layers 

• Formation of the lower uterine segment 

• Softening and effacement of the cervix

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Dimensions at term

At term the uterus is now:

• 30 cm long, 22.5 cms wide and 20cms deep.

• There is a 20 fold increase in weight from approx 60 gms to 900-1100gms.

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Changes in shape & position

• Uterus is expected to follow a predicted rate of growth during pregnancy

• This is only reliable indicator for the first 20 weeks

• Routine symphysis-fundal height and result plotted on customised growth charts now the norm

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Changes after implantation

• Changes in shape and growth are not uniform. 

• The embedded blastocyst does not require much space. 

• Upper part of uterus enlarges due to the influence of oestrogen. 

• Uterus becomes globular in shape until about 20 weeks. 

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Changes after 12 weeks

  • Uterus risen out of the pelvis and become an abdominal rather than pelvic organ. 

• No longer anteverted or anteflexed. 

• Becomes upright and often inclines & rotates to the right. 

• This is known as the right obliquity of the uterus & increases as the pregnancy progresses. 

• Fundus can now be palpated abdominally. 

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Changes 20 weeks onwards

• Uterus then becomes pear shaped until term. 

• Uterus changes shape & size to accommodate growing fetus

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Changes 30 weeks onwards

At this time the lower uterine segment is identified. 

• Complete by approx 36 weeks. 

• LUSCS – safest incision. 

• At 36 weeks the fundus reaches maximum height at the xiphistrenum 

• May then get reduction in uterine size `lightening’ when fetal head enters the pelvis

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Development of the Decidua

• In pregnancy the endometrium is now called the Decidua 

• Divided into 3 sections: 

• Decidua Capsular: is over the embroyo 

• Decidua Basalis – trophoblast rests on this. 

• Decidua Parietalis is the rest of the uterine cavity. 

• Decidua Vera is the Capsular & Parietalis.

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Development of the Decidua

• In pregnancy the endometrium is now called the Decidua 

• Divided into 3 sections: 

• Decidua Capsular: is over the embroyo 

• Decidua Basalis – trophoblast rests on this. 

• Decidua Parietalis is the rest of the uterine cavity. 

• Decidua Vera is the Capsular & Parietalis.

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Growth and development of muscle layers

• Composed of what type of muscle? 

• What are the 3 layers of muscle? Layers more differentiated in pregnancy. 

• Myometrium undergoes exceptional growth throughout pregnancy. 

• By 2 mechanisms. 

• Initially the muscle grows by Hyperplasia (Increasing cell numbers) 

• This happens under the influence of oestrogen. 

• By the 4th month the uterine wall has thickened from 10mm to 25mm. 

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The myometrium

• Subsequent growth is due to Hypertrophy (stretching) by uterine distension. 

• Muscle cells increase markedly in length 

• Length 10 times and width about 3 times. 

• The distribution of smooth muscle varies throughout the length of the uterus 

• Smooth muscle density is highest in the fundus – why?

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Formation of the lower uterine segment

• Lower segment is formed from the 7mm isthmus in the non pregnant uterus. 

• Formed at about 28 – 30 weeks of pregnancy. 

• Hence LUSCS. 

• Previously used the classic incision.

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Softening of the cervix

• Consistency of the cervix changes to become softer in preparation for labour. 

• This is due to increased vascularity and water content. 

• Change occurs in the collagen fibres. 

• Presence of proteoglycans which coat the collagen fibres and modify their physical properties, determining the water content of the tissue. 

• At term 90% of cervix is water. 

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Effacement

• Changes from a 2.5cm long tubular closure to a wide-funnelled canal with very thin edges that is continuous with the rest of the uterine structure. 

• In primigravida women, this shape change occurs in 2 distinct stages.Effacement is the shortening, or thinning, of a tissue. 

• Throughout pregnancy, the cervix is tightly closed and protected by a plug of mucus. 

• When the cervix effaces, the mucus “show” plug is loosened and passes out of the ******. 

• As effacement takes place, the cervix then shortens, or effaces, pulling up into the uterus and becoming part of the lower uterine wall. 

• Effacement is measured in percentages, from zero percent (not effaced at all) to 100 percent, which indicates a paper-thin cervix. 

 

• Effacement is followed by cervical dilation. 

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Uterine activity

• In a multigravida woman effacement and dilatation can occur together. 

• Gradual build up in uterine activity from Braxton Hicks contractions to commencement of labour. 

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Application to Practice

• A/N examination. 

• Induction of labour – Bishop’s score 

• Vaginal examinations 

• APH/Placenta Praevia 

• Management of 3rd

 stage of labour 

• PPH (postp -partum haemorrhage)

• P/N examination 

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