Folic acid supplementation in pregnancy to prevent preterm birth: A systematic review and meta-analysis of randomized controlled trials

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Review

Folic

acid

supplementation

in

pregnancy

to

prevent

preterm

birth:

a

systematic

review

and

meta-analysis

of

randomized

controlled

trials

Gabriele

Saccone

a

,

Vincenzo

Berghella

b,

*

a_Department_of_{Neuroscience,}_Reproductive_Sciences_and_Dentistry,_School_of_Medicine,_University_of_Naples_Federico_II,_Naples,_Italy

b_Division_of_{Maternal-Fetal}_Medicine,_Department_of_Obstetrics_and_Gynecology,_Sidney_Kimmel_Medical_College_of_Thomas_Jefferson_University,_{Philadelphia,}

USA

Contents

1. Materialsandmethods... 76

Acknowledgements... 80

References... 80

Introduction

Pretermbirth(PTB)remainsthenumberonecauseofperinatal mortalityinmanycountries,includingtheUS[1].PriorPTBisone ofthemostimportantriskfactorsforPTB;however,mostofthese PTBsoccurinwomenwithoutapriorPTB[2].

Folic acid (FA) is a water-soluble vitamin of the B group.

Data from observational studies showed that FA, which is

commonly used to prevent neural tube defects (NTDs) [3], mayhavea rolein thepreventionofpregnancy complications such as PTB,small forgestational age,preeclampsia andmay leadtoprolongationof pregnancy[4–7],However,theefﬁcacy of FA supplementation in reducing the risk of PTB is still unclear[4–33].

Theaimofthismeta-analysiswastoevaluatetheefﬁcacyofFA in decreasing the incidence of PTB in asymptomatic singleton gestationswithoutpriorPTB.

ARTICLE INFO

Articlehistory:

Received18November2015

Receivedinrevisedform15January2016 Accepted29January2016 Keywords: Pretermdelivery Folate Diet Supplementation Prematurity ABSTRACT

Folicacid(FA)mayhavearoleinthepreventionofpregnancycomplications.However,theefﬁcacyofFA

supplementationinreducingtheriskofpretermbirth(PTB)isstillunclear.Theaimofthissystematic

reviewwithmeta-analysiswastoevaluatetheefﬁcacyoffolicacidsupplementationduringpregnancy

topreventpretermbirth (PTB).Theresearchprotocolwas designedapriori,deﬁning methodsfor

searchingtheliteratureinelectronicdatabases,includingandexaminingarticles,andextractingand

analyzingdata.Weincludedallrandomizedtrials(RCTs)ofasymptomaticsingletongestationswithout

priorPTBwhowererandomizedtoprophylactictreatmentwitheitherFAsupplementationorcontrol

(placeboornotreatment).TheprimaryoutcomewastheincidenceofPTB<37weeks.Fiverandomized

trials including5,332 asymptomaticsingleton gestationswithout prior PTB wereincluded in the

analysis.WomenwhoreceivedFAsupplementationhadasimilarrateofPTB<37weeks(22.6%vs22.9%;

RR0.99,95%CI0.82 1.18),PTB<34weeks(7.1%vs8.7%;RR0.77,95%CI0.55 1.09)andofpreterm

prematureruptureofmembranes(2.4%vs2.9%;RR0.81,95%CI0.44 1.50)comparedwithcontrol

group.Regardingneonataloutcomewefoundnosigniﬁcantdifferencesinbirthweight(meandifference

85.58g,95%CI-55.17 226.34),lowbirthweight(21.0%vs15.1%;RR0.79,95%CI0.49to1.28)and

perinataldeath(2.9%vs2.4%;RR0.90,95%CI0.60 1.34).Insummary,FAsupplementationduring

pregnancydoesnotpreventPTB<37weeks.DailyFAsupplementationremainsthemostimportant

interventiontoreducetheriskofneuraltubedefects.

ß2016ElsevierIrelandLtd.Allrightsreserved.

* Correspondingauthorat:DepartmentofObstetricsandGynecology,Divisionof Maternal-FetalMedicine,ThomasJeffersonUniversity,833ChestnutStreet,First Floor,Philadelphia,PA19107,USA.Tel.:+12159557996;fax:+12155036619.

E-mailaddress:vincenzo.berghella@jefferson.edu(V.Berghella).

ContentslistsavailableatScienceDirect

European

Journal

of

Obstetrics

&

Gynecology

and

Reproductive

Biology

j o urn a l hom e pa ge : ww w. e l s e v i e r. c om/ l o ca t e / e j ogr b

http://dx.doi.org/10.1016/j.ejogrb.2016.01.042

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1. Materialsandmethods

Theresearchprotocolwasdesignedapriori,deﬁningmethodsfor searching the literature, including and examining articles, and extractingandanalyzingdata.Searcheswereperformedin MED-LINE,Scopus,ClinicalTrials.gov,the PROSPEROInternational Pro-spectiveRegisterofSystematicReviews, EMBASE,Scieloandthe CochraneCentralRegisterof Controlled Trialswith the useof a combinationofkeywordsandtextwordsrelatedto‘‘micronutrients supplementation,’’‘‘folicacid,’’‘‘pregnancy,’’‘‘folate’’and‘‘preterm birth’’ from inception of each database to October 2015. No restrictionsforlanguageorgeographiclocationwereapplied.

We included all randomized trials (RCTs) of asymptomatic

singleton gestations who were randomized to prophylactic

treatment with either FA supplementation or control (either placebo or no treatment). Only trials on singleton gestations withoutpriorPTBwereincluded.Exclusioncriteriaincluded quasi-randomizedtrials(i.e.trialsinwhichallocationwasdoneonthe basisofapseudo-randomsequence,e.g.odd/evenhospitalnumber or date of birth, alternation); trials in women with multiple gestations;priorPTB;FAgivenalsotocontrols;trialswithonly biochemical outcomes available; and trials evaluating other micronutrientsupplementationinadditiontoFA.

Before data extraction, the review was registered with the PROSPERO International Prospective Register of Systematic Reviews(registrationNo.:CRD42014013874).Themeta-analysis

was performed following the Preferred Reporting Item for

SystematicReviewsandMeta-analyses(PRISMA)statement[34]. Dataabstractionwascompletedbytwoindependent investi-gators(GS,VB).Eachinvestigatorindependentlyabstracteddata fromeachstudyandanalyzeddataseparately.Differenceswere reviewed,and furtherresolvedbycommonreviewoftheentire data.Allauthorswerecontactedformissingdataifpossible.

Theriskofbiasineachincludedstudywasassessedbyusingthe criteriaoutlinedintheCochraneHandbookforSystematicReviews ofInterventions[35].Sevendomainsrelatedtoriskofbiaswere assessed in each included trial since there is evidence that theseissues are associated withbiased estimates of treatment effect:1)randomsequencegeneration;2)allocationconcealment; 3)blindingofparticipantsandpersonnel;4)blindingofoutcome assessment;5)incompleteoutcomedata;6)selectivereporting; and7)otherbias.Reviewauthors’judgmentswerecategorizedas ‘‘lowrisk’’,‘‘highrisk’’or‘‘unclearrisk’’ofbias[35].Riskofbiaswas assessed by authors independently (GS, VB). Differences were resolvedbyconsensus.

All analyseswere done usinganintention-to-treat approach, evaluatingwomenaccordingtothetreatmentgrouptowhichthey wererandomlyallocatedintheoriginaltrials.Theprimaryoutcome wastheincidenceofPTB<37weeks.SecondaryoutcomeswerePTB <34weeks,spontaneousPTB(sPTB)<37weeks,sPTB<34weeks, gestationalage(GA)atdelivery,latency,pretermprematurerupture of membranes (PPROM)and neonatal outcomes including birth weight,lowbirthweight(LBW),admissiontoneonatalintensive care unit (NICU), neonatal respiratory distress syndrome (RDS), bronchopulmonarydysplasia(BPD), intraventricularhemorrhage (IVH),necrotizingenterocolitis(NEC),neonatalsepsisandperinatal death. We planned subgroup analysis including RCTs with FA supplementationofmorethan1mgdaily.

Thedataanalysiswascompletedindependentlybyauthors(GA, VB)usingReviewManager5.3(Copenhagen:TheNordicCochrane Centre,Cochrane Collaboration, 2014). The completed analyses werethencompared,andanydifferencewasresolvedwithreview oftheentiredataandindependentanalysis.Statistical heteroge-neitybetweenstudieswasassessedusingtheHigginsI2_statistic._In

caseofstatisticallysigniﬁcantheterogeneity(I2

0%),therandom effectsmodelof DerSimonianand Lairdwasusedtoobtainthe

pooled relative risk (RR) estimate, otherwise in case of no inconsistencyintheriskestimates(I2=0%)a fixedeffectmodel wasperformed[35].ThesummarymeasureswerereportedasRR with95%confidenceinterval(95%CI).Potentialpublicationbiases werestatisticallyassessedbyusingBegg’sandEgger’stests[35]. p-value<0.05wasconsideredstatisticallysignificant.

Results

Weinitiallyidentiﬁed26trialsonFAsupplementationduring pregnancy [8–33]. No similar systematic reviews were found duringthesearchprocess.

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Twenty-one were excluded [13–33]. Two were excluded becausetheywerequasi-randomized trial[13,14];onebecause itevaluatedthetreatmentalsointwins[15];twobecausethey usedalsoothermicronutrients[18,19];11becauseonly biochem-ical outcomes were available [16,17,20–28]; one because it

compared pregnant women with non-pregnant women [29],

and fourbecause theyevaluatedthe samepopulation ofother includedstudies[30–33].Five trials(5,332women),which met inclusion criteria for this meta-analysis,were analyzed [8–12]. Fig.1showstheﬂowdiagram(PRISMAtemplate)ofinformation throughthedifferentphasesofthereview.

Thecharacteristicsofthefiveincludedtrialsaresummarized inTable1.Threestudiesusedironbothintreatmentandincontrol groups.Threestudiesused FA5mg dailyas treatment.Ofthe 5,332asymptomaticsingletongestationswithoutpriorPTBinthe fiveincludedtrials,1,912(35.9%)wererandomizedtoFAgroup, while3,420(64.1%)tocontrolgroup.Publicationbias,assessed statistically by using Begg’s and Egger’s tests, showed no significantbias(p=0.22andp=0.36,respectively).Thequality

ofRCTsincludedinourmeta-analysiswasassessedbyCochrane Collaboration’stool(Fig.2).Threestudiesweredoubleblind.None oftheincludedstudieshadhighriskofbiasin‘‘reportingbias’’and ‘‘attritionbias’’.

ThemeanofGAatrandomizationwasabout18weeksinboth groups.WefoundnodifferencesinGAatdelivery(meandifference 0.10days,95%CI-0.05 0.25)andinlatency(meandifference-0.40, 95%CI-0.87 0.19)betweenthetwogroups.Womenwhoreceived FAsupplementationhadasimilarrateofPTB<37weeks(22.6%vs 22.9%;RR0.99,95%CI0.82 1.18;onestudy;Fig.3),PTB<34weeks (7.1%vs8.7%;RR0.77,95%CI0.55 1.09;onestudy)andofPPROM (2.4%vs2.9%;RR0.81,95%CI0.44 1.50;onestudy)comparedwith controlgroup.Regardingneonataloutcomewefoundnosignificant differences in birth weight (mean difference 85.58g, 95% CI -55.17 226.34;fivestudies),LBW(21.0%vs15.1%;RR0.79,95%CI 0.49 1.28;fourstudies)andperinataldeath(2.9%vs2.4%;RR0.90, 95% CI 0.60 1.34;two studies; Fig.4) (Table2). No datawere availableaboutsPTB<37weeks,sPTB<34weeks,NICU,RDS,BPD, IVH, NEC and neonatal sepsis. We also found no significant

Table1

Descriptivedataofincludedtrials.

Studylocation Years Numberofpatients atrandomization

Intervention Control Blinding* Primary outcome Baumslag19708

SouthAfrica 1968 1969 128(65vs63) FA5mgdailyand200mgiron 200mgirononly Singleblind RateofLBW Fletcher19719

UnitedKingdom 1967 1969 643(321vs322) FA5mgdailyand200mgiron 200mgirononly Doubleblind Biochemical outcomes Iyengar197510 _India _N/A ₂₃₀₍₁₃₄_vs₉₆₎ _FA_0.5_mg_daily_and₆₀_mg_iron ₆₀_mg_iron_only _Double_blind _Birth_weight

Charles200511

UnitedKingdom 1966 1967 2365(463vs1902) FA5mgdaily Placebo Doubleblind Biochemical outcomes Christian200912

Nepal 1998 2001 1966(929vs1037) FA0.4mgdailyandVitaminA (1000gretinolequivalents)

VitaminA(1000g retinol

equivalents)

Singleblind Pretermbirth

Total 5332(1912vs3420)

Dataarepresentedastotalnumber(ninterventionvscontrol);FA:folicacid;mg:milligrams;LBW:lowbirthweight;N/A:notavailable.Dosesofothermicronutrientssuch asironorvitaminAwereidenticalininterventionandcontrolgroups.

*Singleblind,blindingofoutcomeassessment;Doubleblind,blindingofoutcomeassessmentaswellasblindingofparticipantsandpersonnel

Fig.2.Assessmentofriskofbias.(a)Summaryofriskofbiasforeachtrial;Plussign:lowriskofbias;minussign:highriskofbias;questionmark:unclearriskofbias.(b)Risk ofbiasgraphabouteachriskofbiasitempresentedaspercentagesacrossallincludedstudies.

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differencesintheprimaryandsecondaryoutcomesassessedinthe subgroupanalysis(Table3).

Comment

Thismeta-analysisoftheﬁveRCTsevaluatingtheefﬁcacyofFA

supplementation in preventing PTB in 5,332 asymptomatic

singletongestationswithoutpriorPTBshowsthatFA supplemen-tation is not associated with prevention of PTB or improved neonataloutcomes.

Priorevidencefromobservationalstudiesshowedthat,compared withstandardlow-dosesupplementation,additionalfolicacidmay prolonggestation,especiallyifstartedpreconception[4–7]. Biologi-cally,theauthorspointtoanti-inﬂammatorymechanism[4,7].There areseveralhypothesesthatcouldlinkFAandprevention ofPTB.

First, periconception FA supplementation may influence early placentationprocessesand placentaldevelopmentregulatingthe secretionofmatrixmetalloproteinases[36].Second,micronutrient statusatthetimeofimplantationandhomocysteinelevelscould havearoleininflammation[37];infactearlyPTBisoftencausedby intrauterine infection and inflammation [38]. Furthermore, an experimental study on mice showed that FA supplementation duringpregnancypreventsPTB,fetaldeathandintrauterinegrowth restrictionbyananti-inflammatorymechanism[39].

GiventhatFAisrecommendedforallwomenduringpregnancy [3],focusingonwhethertheuseofhigherdoseofFAleadstofurther improved outcomes such as PTB prevention would be helpful. However,wefoundno RCTsevaluatingtheefﬁcacyofhigh-dose compared with low-dose FA; two RCTs are currently ongoing [40,41]. However, even in our subgroup analysis, high-dose FA

Fig.3.Forestplotfortheriskofpretermbirthlessthan37weeks.FA,folicacid;PTB,pretermbirth;CI,conﬁdenceinterval;M-H,Mantel-Haenszeltest.

Fig.4.Forestplotfortheriskofperinataldeath.FA,folicacid;Death,perinataldeath;CI,conﬁdenceinterval;M-H,Mantel-Haenszeltest.

Table2

Primaryandsecondaryoutcomes. GAat randomization mean(weeks) GAat deliverymean (weeks) Latency mean (days) PTB<37 weeks PTB<34 weeks

PPROM BirthWeight, mean(grams)

LBW Perinatal death

Baumslag19708

26vs26 N/A N/A N/A N/A N/A 1547vs1520 4/65(6.2%) vs19/63 (30.2%)

N/A

Fletcher19719 _N/A _N/A _N/A _N/A _N/A _N/A ₃₃₃₀_vs₃₃₃₀ _24/321_(7.5%)

vs16/322 (5.0%)

N/A

Iyengar197510

N/A N/A N/A N/A N/A N/A 2890vs2650 N/A N/A

Charles200511

18vs18 40vs40 158vs158 N/A N/A N/A 3319vs3299 20/459(4.4%) vs115/1890 (6.1%) 8/459(1.7%) vs25/1890 (1.3%) Christian200912 ₁₁_vs₁₁ ₃₈_vs₃₈ ₁₉₀_vs₁₉₁ _175/776 (22.6%) vs201/878 (22.9%) 52/776(7.1%) vs76/878 (8.7%) 17/715 (2.4%) vs24/822 (2.9%) 2587vs2587 262/628 (41.7%) vs297/685 (43.4%) 28/777(3.6%) vs40/876 (4.6%) Total 18vs18 39vs39 160vs161 175/776 (22.6%) vs201/878 (22.9%) 52/776(7.1%) vs76/878 (8.7%) 17/715 (2.4%) vs24/822 (2.9%) 2842vs2796 310/1473 (21.0%) vs447/2960 (15.1%) 36/1236 (2.9%) vs65/2766 (2.4%) Meandifference (95%CI)orRR (95%CI) 0.11weeks (95%CI-0.12 to0.30) 0.10days (95% CI-0.05 0.25) -0.40days (95%CI -0.87 0.19) 0.99(95% CI0.82 1.18) 0.75(95% CI0.53 1.04) 0.81(95%CI 0.44to1.50) 85.58grams (95%CI -55.17 226.34) 0.79(95% CI0.49 1.28) 0.92(95% CI0.58 1.46)

Dataarepresentedasnumberinterventionvsnumbercontrolorastotalnumber(ninterventionvscontrol);GA:gestationalage;PTB:pretermbirth;LBW:lowbirthweight; PPROM:pretermprematureruptureofmembranes;N/A:notavailable;RR:riskratio;CI:conﬁdenceinterval;RCTs:randomizedcontrolledtrials;FA:folicacid.Neonataland deliveryoutcomeswerereferredtolive-bornonly

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supplementation was not associated with prevention of PTB comparedwithcontrol,butdatawerelimited.

Another meta-analysis has evaluated the efﬁcacy of FA in preventionofPTB. However,this meta-analysisdidnotexclude RCTswithFAascontrol,RCTswithmultiplegestations,RCTson women withprior PTB, and RCTsevaluating also other

micro-nutrients. This review found no conclusive beneﬁt of FA

supplementationduringpregnancyonpregnancyoutcomes[42]. OneofthestrengthsofourstudyisinclusionofRCTdataonFA supplementationduringpregnancy in a speciﬁcpopulation, i.e. asymptomatic singleton gestations without prior PTB. This populationrepresents,inallcountries,usuallyabout90%ormore ofthepregnantpopulation.Wechosetolimitinclusionofstudies tothispopulationasotherinterventionstopreventPTBhavebeen shown to have different results in different populations of pregnant women. For example, progesterone supplementation, aswellas cerclage,hasdifferingeffects in populationssuchas singleton without prior PTB versus singletons with prior PTB versusmultiplegestations[43–52].Anotherstrengthofourstudy isthatweplannedasubgroupanalysistoevaluatejusthigh-dose FAsupplementation.Oneauthorprovidedadditionaland unpub-lisheddatafromtheoriginaltrial[12].

Limitationsofourstudyareinherenttothelimitationsofthe includedRCTs.Mostoftheincludedstudieswereconductedduring

the 1970s, so that we found poor compliance with random

allocation.NoneofthestudiesrandomizedFApreconceptionally. Only one RCT had PTB as primary outcome. The dosage of FA

differed somewhat among studies. While no included study

mentionedthatwomenenrolledhadapriorPTB,thischaracteristic wasnotalwaysreported.ThestudybyChristianetal.istheonly onethatcontributedtotheprimaryoutcome,anditwasconducted inNepalwhichisauniquelow-resourcesetting.Thislimitation raisesthequestionofexternalgeneralizabilitytothecurrentUS population.Pretermdeliveryrateswereveryhigh(about23%)for asymptomaticsingletongestations withoutpriorPTB, but were basedontheChristianstudyalone.

Theremaybeotherexplanationsforthefactthatourdatashow no association between FA supplementationand preventionof PTB.First,FAsupplementationmayhavebeengiventoolate.The meanGAatrandomizationwasabout18weeks,andmostother interventionstoprevent PTB,suchas progesterone, cerclageor omega-3,haveusuallyshownefficacywhenimplementedearlier [43,44,50,51].Second,itcouldbethatFAsupplementationisonly beneficialinwomenwithFAdeficiency,avariablewhichwasnot evaluatedin theincludedRCTs.Finally,itcouldalsobethat FA supplementationistrulynoteffectiveinpreventingPTB.

Insummary,FAsupplementationcannotbecurrently recom-mendedsolelyforpreventionofPTB.Giventhelimitationsofthis meta-analysis, further large, well-designed, placebo-controlled trials are needed.Daily FA supplementation remains the most importantinterventiontoreducetheriskofNTDs.

Conﬂictsofinterest

Theauthorsreportnoconﬂictofinterest

Funding

Noﬁnancialsupportwasreceivedforthisstudy. Acknowledgement

WethankDrChristianforprovidingadditionaldatafromtheir trial[12].

References

[1]BhuttaZA,DasJK,BahlR,etal.Canavailableinterventionsendpreventable deathsinmothers,newbornbabies,andstillbirths,andatwhatcost?Lancet 2014;384:347–70.

[2]SpongCY.Predictionandpreventionofrecurrentspontaneouspretermbirth. ObstetGynecol2007;110:405–15.

[3]De-RegilLM,Pena-RosasJP,Fernandez-GaxiolaAC,Rayco-SolonP.Effectsand safetyofpericonceptionaloralfolatesupplementationforpreventingbirth defects.CochraneDatabaseSystRev2015;12:CD007950.

[4]BukowskiR,MaloneFD,PorterFT,etal.Preconceptionalfolate supplementa-tionandtheriskofspontaneouspretermbirth:acohortstudy.PLOSMed 2009;6:61–5.

[5]CatovJ,BodnarL,OlsenJ,OlsenSF,NohrE.Periconceptionalmultivitaminuse andriskofpretermorsmall-for-gestational-agebirthsintheDanishNational BirthCohort.AmJClinNutr2001;94:906–12.

[6]LiZ,YeR,ZhangL,LiH,LiuJ,RenA.Periconceptionalfolicacid supplementa-tionandtheriskofpretermbirthinChina:alargeprospectivecohortstudy.Int JEpidemiol2014;43:1132–9.

[7]SacconeG,SarnoL,RomanA,DonadonoV,MaruottiGM,MartinelliP. 5-methyl-tetrahydrofolateinpreventionofrecurrentpreeclampsia.JMatern FetalNeoantalMed2015;17(March):1–5.

[8]BaumslagN,EdelsteinT,MetzJ.Reductionofincidenceofprematuritybyfolic acidsupplementationinpregnancy.BrMedJ1970;1:16–7.

[9]FletcherJ,GurrA,FellinghamFR,etal.Thevalueoffolicacidsupplementsin pregnancy.JObstetGynaecolBrCwlth1971;78:781–5.

[10]IyengarL,RajalakshmiK.Effectoffolicacidsupplementonbirthweightof infants.AmJObstetGynecol1975;122:332–6.

[11]CharlesDH,NessAR,CampbellD,SmithGD,WhitleyE,HallMH.Folicacid supplementsinpregnancyandbirthoutcome:re-analysisofalarge random-izedcontrolledtrialandupdateofCochranereview.PaediatrPerinat Epide-miol2005;19:112–24.

Table3

Primaryandsecondaryoutcomesinsubgroupanalysisincludingtrialswithfolicacidsupplementationof1mgdaily. GAat randomization mean(weeks) GAat deliverymean (weeks) Latency mean(days) PTB<37 weeks BirthWeight, mean(grams) LBW Perinataldeath Baumslag19708

26vs26 N/A N/A N/A 1547vs1520 4/65vs19/63 N/A Fletcher19719

N/A N/A N/A N/A 3330vs3330 24/321vs

16/322 N/A Charles200511 18vs18 40vs40 158vs158 N/A 3319vs3299 20/459vs115/1890 8/459vs 25/1890 Total 21vs21 40vs40 158vs158 N/A 2433vs2409 48/845(5.7%)vs 150/2275(6.6%) 8/459(1.7%)vs 25/1890(1.3%) RR(95%CI) Mean difference0.50 week(95%CI -0.52 0.30) Mean difference0.10 days(95% CI-0.05 0.25) Mean difference-0.70 days(-0.80 0.60) N/A Mean difference81.29 grams(95%CI -57.24 219.83) 0.66(95% CI0.27 1.63) 1.32(95% CI0.60 2.90)

Dataarepresentedasnumberinterventionvsnumbercontrolorastotalnumber(ninterventionvscontrol);GA:gestationalage;PTB:pretermbirth;LBW:lowbirthweight; PPROM:prematureruptureofmembranes;N/A:notavailable;RR:relativeRisk;CI:conﬁdenceinterval;RCTs:randomized-controlledtrials;FA:folicacid.Neonatal outcomeswerereferredtoalive-bornonly

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[12]ChristianP,KhatrySK,LeClergSC,DaliSM.Effectsofprenatalmicronutrient supplementationoncomplicationsoflaboranddeliveryandpuerperal mor-bidityinruralNepal.IntJGynaecolObstet2009;106:3–7.

[13]RaePG,RobbPM.Megaloblasticanaemiaofpregnancy:aclinicaland labora-torystudywithparticularreferencetothetotalandlabileserumfolatelevels.J ClinPathol1970;23:379–91.

[14]ButtafuocoJP.Preventionofmegaloblasticanaemiainpregnancybyfolicacid. Lancet1963;1:56–9.

[15]FlemingAF,PathMC,HendrickseJP,AllanNC.Thepreventionofmegaloblastic anaemia in pregnancy in Nigeria. J Obstet Gynaecol Br Commonw 1968;75:425–32.

[16]ChanarinI,RothmanD,BerryV.Irondeﬁciencyanditsrelationtofolic-acid statusinpregnancy:resultsofaclinicaltrial.BrMedJ1965;1:480–5.

[17]ChanarinI,RothmanD,WardA,PerryJ.Folatestatusandrequirementin pregnancy.BrMedJ1968;2:390–4.

[18]GilesPFH,HarcourtAG,WhitesideMG.Theeffectofprescribingfolicacid duringpregnancyonbirth-weightanddurationofpregnancy.Adoubleblind trial.MedJAust1971;2:17–21.

[19]LiraP,BarrenaN,ForadoriA,GormazG,GrebeG.Deﬁcienciadefolatosenel embarazo: efecto de una suplementacion con acido folico. Sangre 1989;34:24–7.

[20]DawsonDW,More JR, Aird DC.Preventionof megaloblastic anaemiain pregnancybyfolicacid.Lancet1962;2:1015–8.

[21]ChisholmM.Acontrolledclinicaltrialofprophylacticfolicacidandironin pregnancy.JObstetGynecolBritCwlth1966;73:191–6.

[22]WilloughbyML,JewellFG.Folatestatusthroughoutpregnancyandin post-partumperiod.BrMedJ1968;4:356–60.

[23]HibbardBM,HibbardED.Theprophylaxisoffolatedeﬁciencyinpregnancy. ActaObstetGynecolScand1968;48:339–48.

[24]BalmelliGP,HuserHJ.FolicaciddeﬁciencyinpregnantwomeninSwitzerland. SchweizMedWochenschr1974;104:351–6.

[25]ColmanN,LarsenJ,BarkerM,BakerA,GreenR,MetzJ.Preventionoffolate deﬁciencyby foodfortiﬁcation III.Effect inpregnantsubjects ofvarying amountsofaddedfolicacid.AmJClinNutr1975;28:465–70.

[26]TriggKG,RendallEJC,JohnsonA,FellinghamFR,PrankerdTAJ.Folate supple-mentsduringpregnancy.JRCollGenPract1976;26:228–30.

[27]BatuAT,ToeT,PeH,NyuntKK.Aprophylactictrialofironandfolicacid supplementsinpregnantBurmesewomen.IsrJMedSci1976;12:1410–7.

[28]TaylorDJ,PhillipsP,LindT.Puerperalhaematologicalindices.BrJObstet Gynaecol1981;88:601–6.

[29]GregoryJF,CaudillMA,OpalkoFJ,BaileyLB.Kineticsoffolateturnoverin pregnantwomen(secondtrimester)andnonpregnantcontrolsduringfolic acidsupplementation:stable-isotopiclabelingofplasmafolate,urinaryfolate andfolatecatabolitesshowssubtleeffectsofpregnancyonturnoveroffolate pools.JNutr2001;131:1928–37.

[30]EdelsteinT,Stevens K,Baumslag N,Metz J. Folic acidand vitaminB12 supplementationduringpregnancyinapopulationsubsistingonasuboptimal diet.JObstetGynaecolBritCwlth1968;75:1337.

[31]ChristianP,WestKP,KhatrySK,etal.Effectsofmaternalmicronutrient supplementationonfetalloss andinfantmortality:acluster-randomized trialinNepal.AmJClinNutr2003;78:1194–202.

[32]Christian P,KhatrySK, PradhanEK, et al.Effectof alternativematernal micronutrientsupplementsonlowbirthweightinruralNepal:adouble blindrandomizedcommunitytrial.BMJ2003;326:571–8.

[33]ChristianP,DarmstadtGL,WuL,etal.Theeffectofmaternalmicronutrient supplementationonearlyneonatalmorbidityinruralNepal:arandomized, controlles,communitytrial.ArchDisChild2008;93:660–4.

[34]MoherD, LiberatiA,TetzlaffJ,AltmanDG.Preferredreportingitemsfor systematicreviewsandmeta-analyses:thePRISMAstatement.JClin Epide-miol2009;62:1006–12.

[35]Higgins JPT,Altman DG, Sterne JAC. Cochranehandbook for systematic reviewsofinterventions,version5.1.0(updateMarch2011).TheCochrane Collaboration,2011.Availableat:www.cochrane-handbook.org.(Accessedon November15,2015).

[36]WilliamsPJ,BulmerN,InnesA,PipkinFB.Possiblerolesforfolicacidinthe regulationoftrophoblastinvasionandplacentaldevelopmentinnormalearly humanpregnancy.BiolReprod2011;84:1–10.

[37]VanMil NH, Oosterbaan M,SteegerstheunissenRPM. Teratogenicity and underlyingmechanismofhomocysteineinanimalmodels:areview.Reprod Toxicol2010;4:520–31.

[38]GoldenbergR,HauthJC,AndrewsWW.Intrauterineinfectionandpreterm delivery.NewEngJMed2000;342:1500–7.

[39]ZhaoM,ChenY,DongX,etal.Folicacidprotectsagainstlipopolysaccharide inducedpretermdeliveryandintrauterinegrowthrestrictionthroughits anti-inﬂammatoryeffectinmice.PLOSone2013;8:1–5.

[40]WenWS,ChampagneJ,WhiteR,etal.Effectoffolicacidsupplementationin pregnancyonpreeclampsia:thefolicacidclinicaltrialstudy.JPregnancy 2013;2013.

[41]BortolusR,BlomF,FilippiniF,etal.Preventionofcongenitalmalformations andotheradversepregnancyoutcomeswith4.0mgoffolicacid: community-basedrandomizedclinicaltrialinItalyandtheNetherlands.BMCPregnancy Childbirth2014;14:166–8.

[42]LassiZS,SalamRA,HaiderBA,BhuttaZA.Folicacidsupplementationduring pregnancyformaternalhealthandpregnancyoutcomes.CochraneDatabase SystRev2013;3:CD006896.

[43] SocietyforMaternal-FetalMedicinePublicationsCommittee,with assis-tanceofVincenzoBerghella.Progesteroneandpretermbirthprevention: translatingclinicaltrialsdataintoclinicalpractice.AmJObstetGynecol. 2012;206:376–86.

[44]BerghellaV,OdiboAO,ToMS,RustOA,AlthuisiusSM.Cerclageforshortcervix onultrasonography:ameta-analysisoftrialsusingindividualpatient-level data.ObstetGynecol2005;106:181–9.

[45]RomanA,RochelsonB,FoxNS,etal.Efﬁcacyofultrasound-indicatedcerclage intwinpregnancies.AmJObstetGynecol2015;212(6).788.e1–6.

[46]EhsanipoorRM,SeligmanNS,SacconeG,etal.Physicalexamination-indicated cerclage: a systematic review and meta-analysis. Obstet Gynecol 2015;126:125–35.

[47]SacconeG,RustO,AlthuisiusS,RomanA,BerghellaV.Cerclageforshortcervixin twinpregnancies:systematicreviewandmeta-analysisofrandomizedtrials usingindividualpatient-leveldata.ActaObstetGynecolScand2015;94:352–8.

[48]SacconeG,BerghellaV,MaruottiGM,SarnoL,MartinelliP.Omega-3 supplemen-tationduringpregnancytopreventrecurrentintrauterinegrowthrestriction:a systematicreviewandmeta-analysisofrandomizedcontrolledtrials.Ultrasound ObstetGynecol2015;(May):29.http://dx.doi.org/10.1002/uog.14910. [49]Saccone G, Berghella V. Omega-3supplementation to prevent recurrent

pretermbirth:asystematicreviewandmeta-analysis.AmJObstetGynecol 2015;213:135–40.

[50]SacconeG,BerghellaV.Omega-3longchainpolyunsaturatedfattyacidsto preventpretermbirth:asystematicreviewandmeta-analysis.ObstetGynecol 2015;3:663–72.

[51]SacconeG,SacconeI,BerghellaV.Omega-3longchainpolyunsaturatedfatty acids andﬁshoilsupplementationduringpregnancy: which evidence?J MaternFetalNeonatlMed2015;18:1–9.