Should a pregnantwoman scuba dive?

Whether expectant womenshould dive is a question thataffects not only female diversbut also their partners, divebuddies and dive professionals.Most divers can recall fromtheir open water training thatwomen are encouraged to stopdiving during pregnancy, butfew classes go into furtherdetail.

What are the risks of divingwhile pregnant? What is itabout scuba diving that isdangerous for a developingfoetus? The published literatureprovides a foundation for thediscussion.

The Risks of DivingWhile Pregnant

Reviewing the Research

BY HEATHER E. HELD, BS ANDDR. NEAL W. POLLOCK, PHD

David Haas Photo

As with all research, there arelimitations on how much theavailable studies can tell us. For

ethical reasons, experiments withpregnant women are very limited. Moststudies conducted with humans aresurveys, and surveys have weaknesses,most importantly that they are not aseasily controlled as laboratory researchand that they can easily be biased.

A survey of female divers who hadrecently given birth included 69 womenwho had not dived during theirpregnancies and 109 women who had.The nondiving women reported nobirth defects, while the diving womenreported an incidence of 5.5 percent.7To provide perspective, the surveyauthor stated that the latter rate waswithin the normal range for thenational population. The small samplesize and the likelihood of selection biasin those responding to the survey makethe results even more difficult tointerpret.

While surveys can establish correlations,they cannot confirm causalrelationships. In this case, they cannotconfirm that diving caused a defect.To obtain such data, scientists rely onmore highly controlled animal studies.

'Diving' in chambersHyperbaric chambers, which cansimulate the increased pressure ofdiving, have been used to test differentspecies of animals. Those results mustthen be translated to the humanexperience.

Many complex processes occur duringpregnancy, and insults (disruptions ofnormal events) can lead to variedcomplications. Most diving-relatedstudies have addressed the first andthird trimesters of pregnancy. Firsttrimester research has concentrated onthe teratogenic, or birth-defect-causing,effects of hyperbaric oxygen (HBO).Third trimester research has examinedthe effects of decompression sickness(DCS) on the foetus and how diving

TABLE 1: Depth (approx) at oxygen partial pressure limits for humans

and the foetal circulatory systeminteract.

A range of developmentalabnormalities have been associated withhyperbaric exposure. These include lowbirth weights among the offspring ofdiving mothers14,15,26; foetal abortion28;bubbles in the amniotic fluid13,25;premature delivery14; abnormal skulldevelopment11,15,16; malformedlimbs11,15,16; abnormal development ofthe heart16,20; changes in the foetalcirculation2; limb weakness associatedwith decompression illness21; andblindness14.

We expose ourselves to hyperbaricoxygen - that is, oxygen concentratedby pressure - during almost all dives.A safe limit for the partial pressure ofoxygen (PO2) is frequently accepted as1.4 to 1.6 atmospheres of absolutepressure (ATA)19. Table 1 shows thedepth (in msw) where these PO2 levelsare achieved with different breathinggas mixtures.

Rodents, which have large litters andrelatively short gestational periods12,have been used to study the effects ofHBO on developing foetuses. Femalehamsters experiencing untreated DCIhad offspring with severe limb and skullabnormalities.15,16 Pregnant hamstersexperiencing HBO-treated decom-pression illness also bore offspring withdefects, though with less frequency thanthe untreated group15. Neither studyreported noticeable differences inanatomical development betweenoffspring from the nondiving controlgroup and the group that dived withoutdeveloping signs of DCI15,16.

Foetal rat hearts have proven sensitiveto multihour HBO exposure (3.0 ATAfor eight hours), albeit of a magnitudein excess of what humans couldtolerate. In almost half the cases, theseptum, which divides the right and leftsides of the heart, failed to formproperly20. Major blood vessels werepositioned incorrectly just as often,compromising normal circulatorypatterns20.

Another study of HBO-exposed ratsfound no significant differencesbetween offspring from mothers thathad dived and offspring from mothersthat had not dived6. The PO2 in thisstudy (1.3 ATA for 70 minutes) wassignificantly less than that used in theprevious study. The treatmentdifference may explain the dissimilarresults.

Table 2 shows a summary of thetimeline for human foetal development.It appears that hyperbaric exposure canalter the signals foetal tissues rely on tocorrectly orchestrate developmentalprocesses. The nature of theabnormality is influenced by the timingof the insult. It is important to note,however, that exposure will not affectdevelopment in all instances.

Decompression stressThe relative risk of decompressionstress on mother and foetus is anotherquestion for consideration. Givensufficient decompression stress, bloodreturning to the heart from the bodymay contain venous gas emboli (VGEor bubbles)23. Sheep have been studiedfrequently because of the similaritybetween sheep and human placentae.

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TABLE 2: foetal developmeent during pregnancy(modified from WebMD21 and the March of Dimes16 websites)

Foetal sheep whose mothersunderwent decompression dives(following U.S. Navy dive tables)sometimes formed bubbles even whenthe mothers showed no signs ofDCI13,21.

When the ewes did develop signs ofDCI, the foetuses demonstrated evenmore dramatic evidence of affliction.Researchers reported being able to tellthat a foetus had bubbles by detectingearly cardiac arrhythmias21. For thefoetus, these abnormal heartbeatscould be life-threatening. The offspringof some sheep that were dived late inpregnancy showed limb weakness andspinal defects associated with DCI,even when the mother had remainedsymptom-free21.

Scientists have long known that so-called 'silent bubbles' - those notassociated with symptoms - candevelop after diving (note: Dr. AlbertBehnke, a pioneer in modern divingmedicine and physiology research, iscredited for coining this term4). Fullyfunctional lungs are extremely effectivein filtering bubbles from thecirculation. In the foetus, however,most blood bypasses the lungs (via theforamen ovale and ductus arteriosus

shunts), and gas exchange occursthrough the placenta. Thus, pulmonaryfiltration of bubbles does not occurwithin the foetus. This may increase therisk of arterial gas embolism (AGE),with potentially devastatingconsequences.

Foetal circulation requires furtherconsideration. During a series of divesthat exposed ewes to 100 percentoxygen at 3.0 ATA for approximately50 minutes, researchers noticed that thecirculatory shunts began to close whileat depth. Flow through the foramenovale dropped by 50 percent, and theductus arteriosus flow fell to zero oreven reversed direction2.

Once the dives were completed, thecirculation reverted to its usual form,and the researchers did not notice anynegative effects from the temporarychange. Whether the foetus sufferedconsequences that were not obvious tothe researchers was unclear.

The animal study data can be comparedwith human experience. Prematureclosure of the ductus arteriosus duringhuman pregnancy has been associatedwith congestive heart failure1,3,18 andneonatal death3,5,18. Such closure can

unintentionally be induced byprolonged use of indomethacin, a drugcommonly used to halt prematurelabor9. Whether scuba diving couldinduce problematic closure is uncertain,but the possibility should beconsidered.

Practical considerationsIn addition to possible risk to thefoetus, changes in a woman's bodyduring pregnancy might make divingmore problematic. Swelling of themucous membranes in the sinusescould make ear clearing difficult8,10, andnausea may increase discomfort10,27.

The physical aspects must also beappreciated. A woman's growingabdomen could pose a problem infitting suits, buoyancy compensationdevices, weight belts and otherequipment. In addition to the hazardsinherent in poorly fitted gear, divingsimply may not be enjoyable.

DecisionsSifting through the published literaturereveals why there is debate over thetopic. Data are limited and, in manycases, apparently inconsistent. Whilethis makes drawing conclusions moredifficult, it should not be surprising.

Science is very rarely as clear-cut asmight be desired. It is difficult to designan ethical experiment that tests only thevariable of interest and controls for allothers. It is the researcher's job todesign the best experiments possible,and it is the individual's or advocate'sresponsibility to examine the resultsand decide how to best respond tothem.

The question of diving and pregnancyis a difficult one to study since the trendis for women to refrain from divingwhile pregnant 24. Most physicians treatdiving as they would any drug forwhich the evidence with respect topregnancy is incomplete: If there is nota good reason to take it, avoid it.

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Anyone who inadvertently dives whilepregnant, however, may take solace inthe anecdotal evidence from womenreporting repeated diving duringpregnancy without complication.There is certainly insufficient evidenceto warrant termination of a pregnancy.Moreover, if emergency hyperbaricoxygen is required during pregnancy,for example to treat carbon monoxidepoisoning, the evidence suggests thatthe risk to the foetus with treatment islower than without.

The overall picture of the literatureindicates that, while the effect may besmall, diving during pregnancy doesincrease the risk to the foetus, and theconsequences could be devastating toall involved. Appreciating these essentialfactors, the prudent course is to avoiddiving while pregnant. While it ispossible that some diving could becompleted without impact, theabsolute risk of any given exposurecannot be determined from theavailable data. Given the ethicalchallenges of research on diving duringpregnancy and the fact that divingrepresents a completely avoidable riskfor most women, it is unlikely thatstudies will be conducted to establishthe absolute risk in the foreseeablefuture.

About the AuthorsHeather Held completed anundergraduate degree inbiomedical science at Texas A&MUniversity in December 2005. Shewas placed at the Centre forHyperbaric Medicine andEnvironmental Physiology, DukeUniversity Medical Center,Durham, N.C., as a DAN researchintern in the summer of 2006.

Dr. Neal Pollock is a researchphysiologist at the Centre forHyperbaric Medicine andEnvironmental Physiology, DukeUniversity Medical Center,Durham, N.C.

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