Aetiology

Obstetric causes can be divided into those in early and late pregnancy. Conditions that span the entire gestation period have been categorised according to their anatomical system. For simplicity, only conditions that more commonly occur ante-partum (before labour commences), rather than intra- or postpartum, have been included.

Early pregnancy

Ectopic pregnancy

  • Usually diagnosed in the first trimester, most commonly affecting the fallopian tube.[5]

  • Strongly associated with conditions that damage the fallopian tube, such as pelvic inflammatory disease (PID), previous ectopic pregnancy, and previous tubal surgery.

  • Also associated with current IUD users and women whose pregnancy is the result of assisted reproductive technologies.[6]​ Ectopic pregnancy incidence following assisted reproductive technology appears to be decreasing with time.[7][8]​​

Miscarriage

  • Usually occurs before the 12th week, with approximately 50% of all cases of early pregnancy loss due to fetal chromosomal abnormalities.[9][10][11][12]​​​​

  • Other causes include multiple pregnancy, uterine pathology (e.g., fibroids), cytotoxic drugs, radiation, and systemic conditions (e.g., anti-phospholipid syndrome).

Ovarian hyperstimulation syndrome (OHSS)

  • At least 2% of pregnancies in industrialised countries are consequent to advanced fertility treatment such as in vitro fertilisation.[13]

  • Severe OHSS is a potentially life-threatening iatrogenic complication, occurring in approximately 0.5% to 2% of women undergoing gonadotrophin stimulation.[14][15][16]​​​

  • Risk of OHSS is highest in young women with polycystic ovary syndrome or low body mass index.[17]

  • Severe cases of OHSS are characterised by enlarged ovaries, ascites, increased blood viscosity, and renal or hepatic dysfunction.[18]

Late pregnancy

Pre-term labour

  • May be triggered by nephrolithiasis, urinary tract infections (UTIs) (particularly pyelonephritis), haemolysis, elevated liver enzymes, low platelet count (HELLP) syndrome, placental abruption (implicated in up to 10% of premature deliveries), chorioamnionitis, and appendicitis.[19]

  • Acute cystitis may irritate the uterine segment and trigger uterine contractions.

  • More likely to occur with non-operative management of cholecystitis compared with those undergoing cholecystectomy.[20]

Chorioamnionitis

  • Complicates 1% to 5% of full-term pregnancies and is seen in about 25% of pre-term deliveries.[21]

  • Associated organisms include Ureaplasma urealyticum, Mycoplasma hominis, Gardnerella vaginalis, Peptostreptococcus, and Bacteroides species.

  • Risk increased following chorionic villus sampling (CVS) and amniocentesis.

Uterine rupture

  • High risk in patients who have had previous uterine surgery (e.g., caesarean section, myomectomy).[22]

Placental abruption

  • Usually associated with vaginal bleeding.[23][24]

  • Risk factors include maternal age ≥35 years, multi-parity, cigarette smoking, cocaine and other drug use, multiple gestation, chronic hypertension, mild and severe pre-eclampsia, premature rupture of membranes, oligohydramnios, and chorioamnionitis.[23][24]

Haemolysis, elevated liver enzymes, and low platelet count (HELLP) syndrome

  • Characterised by haemolytic anaemia, elevated liver enzymes, and thrombocytopenia.

  • A severe form of pre-eclampsia (sometimes called 'atypical pre-eclampsia’).[25]

Acute fatty liver of pregnancy

  • Rare but life-threatening condition characterised by micro-vesicular fatty infiltration of the liver.

  • Most commonly occurs in the third trimester.[26]

Incarceration of the gravid uterus

  • A rare condition that occurs when the growing uterus becomes entrapped in the pelvis between the pubic symphysis and the sacral promontory, preventing its transition from a pelvic to an abdominal organ.[27]​ If not promptly recognised and treated, incarceration of the gravid uterus is a potentially devastating complication of pregnancy.

  • Occurrence is estimated to be approximately 1 in 3000 pregnancies.[28]

  • Incarceration of the gravid uterus is typically diagnosed in the second trimester (median gestational age of approximately 17 weeks at presentation).[29][30]

Gynaecological

Adnexal masses

  • Incidence of adnexal mass in pregnancy has been reported to be between 0.05% and 6%.[31][32][33][34][35]​​​​​

  • Pelvic masses should be considered cancerous until proved otherwise. The vast majority of adnexal cysts in female reproductive-age patients are benign and resolve spontaneously.[32] Consideration of the possibility of malignancy is, however, prudent. 

  • Adnexal torsion, and its recurrence, is more common during pregnancy.[36][37]​​ Torsion occurs more frequently on the right side than the left.[36]​ Exogenous ovarian stimulation (assisted reproductive technology) is a risk factor for adnexal torsion.[37][38]​​​​

  • Adnexal mass causing abdominal pain (torsion, haemorrhage, and rupture) is more likely to present in the first trimester.​​[39]​​[40]

  • Adnexal masses considered for intervention include those that are:[32][34]

    • persistent, large (>10 cm, present after the first trimester)

    • at increased risk for torsion

    • symptomatic

    • found to have features concerning for malignancy on ultrasound.

  • Ovarian cyst rupture is rare, and may occur in conjunction with torsion.[41][42]

  • Massive haemorrhage into an ovarian cyst (particularly a malignant cyst) occurs infrequently and causes pain similar to that of torsion.

  • Haemorrhage may also result in a corpus luteum cyst.[43][44]​​​ At the time of formation of the corpus luteum, there is always a little bleeding into the follicle that has discharged its ovum. If the bleeding is excessive, the corpus luteum becomes distended with blood. Cysts vary in size from 2.5 to 10 cm.

Fibroids

  • The incidence of fibroids during pregnancy is unknown; rates of between 0.1% and 12.5% have been reported.[45][46]

  • Most growth happens in early pregnancy.[47][48]

  • Cause pain in pregnancy mainly because of red degeneration (necrobiosis), which occurs in 5% to 10% of pregnancies between 12 and 20 weeks' gestation.[45]

  • Pain may also result from torsion of a pedunculated fibroid or from fibroid impaction.[49]

Urological

Urological conditions in pregnancy present a major diagnostic challenge, as the urinary tract undergoes profound anatomical and physiological changes that predispose pregnant women to an increased risk for symptomatic UTIs and kidney stone formation. The ureters become dilated as early as the first trimester and remain distended until after delivery. This is mainly due to compression exerted by the growing uterus on the collecting system above the level of the linea terminalis. This leads to vascular congestion of the upper urinary tract, with consequent urinary stasis. These changes can facilitate aggregation of crystals in the urine and increase the risk of ascending infection. The following conditions may subsequently occur.

UTI

  • Most frequently Escherichia coli (80% to 90% of UTIs during pregnancy)

  • Other gram-negative rods isolated: Proteus mirabilis, Klebsiella pneumonia

  • Incidence increases with low socio-economic status, increasing age, sexual activity, and diabetes mellitus.

Acute pyelonephritis

  • Most common during the second half of pregnancy, as a result of increased ureteral obstruction and urinary stasis[50]

  • Usually uni-lateral and more common in the right kidney

  • E coli is the predominant organism.

Nephrolithiasis

  • Usually occurs during the second and third trimesters

  • Three times more likely in multi-parous women

  • Ureteric stones more common than kidney stones.[51]

Hydronephrosis

  • Common physiological condition in pregnancy; disappears rapidly after birth[52]

  • Most commonly (90%) occurs after the 20th week of gestation, being more pronounced in primigravidae

  • Dilatation only seen above the linea terminalis and is more frequently right sided

  • Compression of the ureters by the uterus (causing hydronephrosis) can result in acute attacks of pain triggered by ureteral obstruction.

Gastrointestinal

A number of common gastrointestinal (GI) conditions can complicate pregnancy and present with abdominal pain. In turn, pregnancy can make the diagnosis of these conditions difficult due to the physiological changes occurring to the body during this time.

Appendicitis

  • Acute appendicitis occurs approximately at a rate of 1:1250 to 1:1500.[53]

  • No single symptom, sign, or laboratory finding is diagnostic for acute appendicitis.[54]

  • Diagnosis complicated by anatomical changes in the location of the appendix as pregnancy progresses.[55]

  • Clinical judgement vital in deciding whether surgical management is needed.[Figure caption and citation for the preceding image starts]: The site of the appendix changes as pregnancy advancesChamberlain G. ABC of antenatal care: abdominal pain in pregnancy. BMJ. 1991;302:390-1394. Used with permission [Citation ends].com.bmj.content.model.assessment.Caption@7f8b6695

Cholecystitis

  • Caused by cholelithiasis in >90% of cases in pregnancy.[56]

  • Cholelithiasis risk is increased by progesterone-induced smooth muscle relaxation of the gallbladder (which promotes stasis of the bile) and elevated levels of oestrogens (which increase the lithogenicity of bile).

Pancreatitis

  • Cholelithiasis and hypertriglyceridaemia may be common causes in pregnancy.[57][58]​​[59]

  • Tends to occur late in the third trimester, possibly due to increased intra-abdominal pressure on the biliary ducts.[60]

Intestinal obstruction

  • Incidence increasing with increased frequency of intra-abdominal surgery, pelvic surgery, and PID.

  • Most commonly caused by simple obstruction, mainly as a result of adhesions (60% to 70% of cases) or volvulus (25%).[2]

  • Rarely caused by intussusception, hernias, and neoplasms.

Traumatic

Traumatic causes of abdominal pain should not be overlooked in pregnant women. Abdominal pain following blunt trauma affects 6% to 7% of pregnancies, 0.3% of which require admission.[61] The most common causes are road traffic accidents, accidental falls, and physical abuse. Splenic rupture may be due to domestic violence.[62]

Musculoskeletal

Although uncommon, a rectus sheath haematoma can be the cause of abdominal pain in pregnancy. Trauma, obesity, straining, coughing, previous abdominal surgery, vascular pathologies, disorders of coagulation, endometriosis of the rectus sheath, Cushing's syndrome, and anticoagulation therapy may contribute to the occurrence of this condition.[63]

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