Featal macrosomia: definition,risk factors, diagnosis, complications and management.

Definition of macrosomia:

Generally to describe a very large fetus or a neonate the term macrosomia is used. According to obstetrical practice weight more than  4 kgs is taken as large baby.

Birth weight exceeding the 90 th percentile of the average body weight of the particular gestational age can be taken as macrosomia.

Birth weight two standard deviations above the mean birth weight of that gestational age, means 97 th percentile of mean birth weight is taken as macrosomia.

If birth weight threshold is taken as 4 kgs, exceeding that weight can be taken as macrosomia.

Different countries use their own criteria to define macrosomia. According to American college of obstetricians and gynecologists it is 4500 gms or more.

Risk factors for macrosomia:

Not all mothers who have delivered macrosomic babies will show risk factors. Only 40% of women with macrosomic fetuses will show some risk factors like:

Maternal diabetes mellitus:

As the baby weight goes above 4 kgs the chances of maternal diabetes is more. But this condition is associated only with a small percentage of macrosomic infants.

The processes how maternal diabetes leads to fetal macrosomia:

 

  1.   Maternal hyperglycemia

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                Hypertrophy and hyperplasia of the fetal islet cells of langerhans

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                             Increased secretion of fetal insulin

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                  Stimulates carbohydrate utilization and accumulation of fat

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     Fetal macrosomia

2. insulin like growth factors I and II are also involved in fetal growth and adiposity.

3.in diabetes maternal free fatty acids(FFA) will be  elevated and these will be excessively transformed to fetus leading to acceleration of triglyceride synthesis and which further lead to adiposity.

In macrosomic fetuses of diabetic mother, shoulder circumference will be more and shoulder circumference to head circumference is also more leading to shoulder dystocia and increased proportion of body fat is also associated with labor dystocia and operative delivery.

With good diabetic control the incidence of macrosomia can be reduced.

Familial:

If patients are of large size, the chances of having large baby is more. This is more true when mother is obese, if her weight is above 300 pounds, the fetal macrosomia can be seen in 30% of cases.

Multiparity:

As the birth order goes on increasing the chances of having big babies are more.

Probably because multipara with increasing age more prone to have obesity, diabetes mellitus. The abdomen becomes lax and the space availability will be more for the baby and compared with first pregnancy she will remain tension free as she already experienced the process of pregnancy previously and there will be less incidence of  hyperemesis gravidarum allowing her to take good diet.

Prolonged pregnancy:

In some cases of prolonged pregnancy the baby growth goes on increasing leading to fetal macrosomia.

In elderly mothers:

Chances of  having medical complications like gestational diabetes are more which may lead to macrosomia.

Tendency to prolongation of pregnancy may also lead to macrosomic babies in case of continued growth.

Male fetus:  

The chances of macrosomia is more common in male fetuses may be because of excessive bone mass.

Previous macrosomic baby:

If mother had a baby more than 4 kgs in previous pregnancy she may the have macrosomic baby in present pregnancy also. May be because the same factors which are influenced in the previous pregnancy repeat in present pregnancy.

Depending on race and ethnicity:

in some parts of the world depending on the average size of the people in that particular country incidence of macrosomic babies will alter.

As in USA the cutoff for macrosomic baby is 4.5 kg, in India the average birth weight is 2.5 to 3.5 kg and more than 4 kg is taken as macrosomia.

Weight gain of the mother:

Weight gain of the mother in the present pregnancy will influence the weigh of the baby. Depending on mother’s pre pregnancy weight the weight gain in pregnancy is advised which should be gradual gain. In case of obese women this should be restricted to 6 to 7 kgs and it can be up to 11 to 12 kgs in non obese women.

Diagnosis of macrosomia:

Exact confirmation of macrosomia can be done only after delivery of the baby.

Though accurately cannot be detected some of the estimations can be done to diagnose the fetal weight before delivery like:

·         Disproportionate increase in uterine size.

·         Fetus feels big and firm with hard skull bones.

·         Fetal weight: symphysio fundal height in cms  - 11 × 155 (in case of engaged head)

              symphysio fundal height in cms - 12 × 155 (in case of unengaged head)

·         If abdominal circumference is more than 100 cms it can be taken as big baby.

·         According to ultrasonography measurements, BPD, AC, HC, and FL birth weight can be calculated.

·         Based on BPD and  AC, baby  with BPD of approximately 9.9 cm with AC of 36 cm can be taken as having weight above 4 kgs.

·         Based on FL and AC, baby with FL of around 8 cm and AC of around 36 cm can be taken as above 4 kg baby.

·         Head to abdomen ratio < 0.9 indicates head to body disproportion with increased risk of shoulder dystocia.

·         Ultrasonography will give fetal weight about ± 15% of actual birth weight.

·         But the obesity of the mother and skill of the sonologist will decide the accuracy of the measurements.

Risks that can occur due to macrosomia:

·         Cephalo pelvic disproportion can occur during labor.

·         Shoulder dystocia can arise especially in diabetic patients.

·         These can lead to prolonged labor or obstructed labor leading to increased chances of operative interference.

·          Maternal  morbidity increases due to:

Chances  of prolonged labor leading to infections, operative deliveries, injuries to the genital tract.

 In  neglected cases even uterine rupture may occur leading to maternal mortality. Postpartum hemorrhage can occur due to either atonicity because of excessive distension of uterus or traumatic due to operative interference.

·         Fetal complications likely to occur are:

Injuries  to babies due to operative interference.

Shoulder dystocia can lead to brachial plexus injury.

Fetal asphyxia and further consequences can occur due to prolonged or obstructed labor.

Meconium aspiration syndrome can occur due to above causes.

Macrosomic babies of diabetic mothers may have hypoglycemia after birth.

If the baby remains obese in future other obesity related disorders can occur.

Management of pregnancy with macrosomic baby:

Induction of labor:

Labor can be induced prophylactically when macrosomia is diagnosed to prevent further growth of the baby. But this is not found to prevent the chances of caesarean delivery or shoulder dystocia.

Elective caesarean delivery:

Caesarean delivery can be planned electively in women having macrosomic baby to prevent delivery complications.

But the risks of caesarean delivery to be kept in mind. Economically also it will create burden to the patient’s family.

But it is acceptable in macrosomic babies of diabetic mothers with fetal weight of > 4.25 to 4.5 kg as the chances of shoulder dystocia is more in these cases.

Prevention of shoulder dystocia:

When the fetal head passes through maternal pelvis but shoulders cannot pass because of very large diameter leading to impaction of anterior shoulder against maternal symphysis pubis that condition is called shoulder dystocia.

In dealing with macrosomic baby the chances of shoulder dystocia to be kept in mind, if it occurs the measures to be taken are:

First  liberal medio lateral episiotomy to be given.

Mc Robert’s manoeuvre: maternal thighs to be abducted and sharply flexed on to her abdomen this process may relieve the impacted anterior shoulder.

 If not then head and neck should be grasped and taken posteriorly and simultaneous supra pubic pressure to be applied by assistant slightly towards fetal chest, by doing this fetal bisacromial diameter can be reduced and anterior shoulder can be rotated obliquely.

Wood’s manoeuvre: if above method also fails general anesthesia to be given, the posterior shoulder to be rotated to anterior position keeping it inside the pelvis by a screwing movement with simultaneous suprapubic pressure in opposite direction.

Zavanelli’s manoeuvre: fetal head to be flexed and replaced within the uterus and baby will be delivered by emergency caesarean section.

In case of dead fetus or live anencephalic fetus which is not able to deliver cleidotomy can be done, that is cutting one or both clavicles to reduce the shoulder girth.

Because of shoulder dystocia brachial plexus injury, birth asphyxia or even fetal death can occur.