RESUMING ADEQUATE SCREENING FOR GESTATIONAL DIABETES MELLITUS DURING THE ONGOING COVID-19 PANDEMIC.

The novel coronavirus disease 2019 (COVID-19) pandemic has had a profound impact on antenatal care, forcing authorities to consider some medical services unessential in the pursuit of avoiding the valid risk of patient contamination. The oral glucose tolerance test (OGTT) has been in some cases overlooked for screening in pregnancy, with potential detrimental consequences in terms of not diagnosing and treating gestational diabetes mellitus (GDM). The number of tests has dropped by 35% in 2020 in our hospital. We make a plea for resuming OGTT at 24-28 weeks gestation at least for women considered at high risk.

Iatrogenic Cushing's syndrome as a consequence of nasal use of Betamethasone spray during pregnancy.

INTRODUCTION: Glucocorticoids (GC) are largely used for their anti-inflammatory and immunosuppressive effects. Until recently "local" administration (inhalation, topical, intra-articular, ocular and nasal) was considered devoid of important systemic side effects, but there is no administration form, dosing or treatment duration for which the risk of iatrogenic Cushing's syndrome (CS) and consequent adrenal insufficiency (AI) can be excluded with certainty. PATIENTS AND METHODS: We present the case of a pregnant woman who developed overt CS with secondary AI in the second trimester of pregnancy. She had low morning plasma cortisol 6.95 nmol/L (normal non-pregnant range 166 - 507) and low ACTH level 1.54 pg/mL (normal range 7.2 - 63.3), suggestive for iatrogenic CS. A thorough anamnesis revealed chronic sinusitis long-term treated with high doses of intranasal betamethasone spray (6 - 10 applications/day, approximately 10 mg betamethasone/week, for 5 months). After decreasing the dose and switching to an alpha-1 adrenergic agonist spray, the adrenal function recovered in a few weeks without manifestations of AI. The patient underwent an uneventful delivery of a normal baby. A review of the literature showed that only a few cases with exogenous CS and consequent AI caused by intranasal GC administration were described, mostly in children, but none during pregnancy. CONCLUSION: Long-term high doses of intranasal GC may induce iatrogenic CS and should be avoided. Low levels of ACTH and cortisol should prompt a detailed anamnesis looking for various types of glucocorticoid administration.

Two-stage approach for prediction of small-for-gestational-age neonate and adverse perinatal outcome by routine ultrasound examination at 35-37 weeks' gestation.

BACKGROUND: Justification of prenatal screening for small-for-gestational-age (SGA) fetuses near term is based on, first, evidence that such fetuses/neonates are at increased risk of stillbirth and adverse perinatal outcome, and, second, the expectation that these risks can be reduced by medical interventions, such as early delivery. However, there are no randomized studies demonstrating that routine screening for SGA fetuses and appropriate interventions in the high-risk group can reduce adverse perinatal outcome. Before such meaningful studies can be undertaken, it is essential that the best approach for effective identification of SGA neonates is determined, and that the contribution of SGA neonates to the overall rate of adverse perinatal outcome is established. In a previous study of pregnancies undergoing routine ultrasound examination at 35 + 0 to 36 + 6 weeks' gestation, we found that, first, screening by estimated fetal weight (EFW) < 10th percentile provided poor prediction of SGA neonates and, second, prediction of > 85% of SGA neonates requires use of EFW < 40th percentile. OBJECTIVES: To examine the contribution of SGA fetuses to the overall rate of adverse perinatal outcome and, to propose a two-stage approach for prediction of a SGA neonate at routine ultrasound examination at 35 + 0 to 36 + 6 weeks' gestation. METHODS: This was a prospective study of 45 847 singleton pregnancies undergoing routine ultrasound examination at 35 + 0 to 36 + 6 weeks' gestation. First, we examined the relationship between birth-weight percentile and adverse perinatal outcome, defined as stillbirth, neonatal death or admission to the neonatal unit for ≥ 48 h. Second, we used a two-stage approach for prediction of a SGA neonate and adverse perinatal outcome; in the first stage, fetal biometry was used to distinguish between pregnancies at very low risk (EFW ≥ 40th percentile) and those at increased risk (EFW < 40th percentile) and, in the second stage, the pregnancies with EFW < 40th percentile were stratified into high-, intermediate- and low-risk groups based on the results of EFW and pulsatility index in the uterine arteries, umbilical artery and fetal middle cerebral artery. Different percentiles of EFW and Doppler indices were used to define each risk category, and the performance of screening for a SGA neonate and adverse perinatal outcome in pregnancies delivered at ≤ 2, 2.1-4 and > 4 weeks after assessment was determined. We propose that the high-risk group would require monitoring from initial assessment to delivery, the intermediate-risk group would require monitoring from 2 weeks after initial assessment to delivery, the low-risk group would require monitoring from 4 weeks after initial assessment to delivery, and the very low-risk group would not require any further reassessment. RESULTS: First, although in neonates with low birth weight (< 10th percentile) the risk of adverse perinatal outcome is increased, 84% of adverse perinatal events occur in the group with birth weight ≥ 10th percentile. Second, in screening by EFW < 10th percentile, the predictive performance for a SGA neonate is modest for those born at ≤ 2 weeks after assessment (83% and 69% for neonates with birth weight < 3rd and < 10th percentiles, respectively), but poor for those born at 2.1-4 weeks (65% and 45%, respectively) and > 4 weeks (40% and 30%, respectively) after assessment. Third, improved performance of screening, especially for those delivered at > 2 weeks after assessment, is potentially achieved by a proposed new approach for stratifying pregnancies into management groups based on findings of EFW and Doppler indices (prediction of birth weight < 3rd and < 10th percentiles for deliveries at ≤ 2, 2.1-4 and > 4 weeks after assessment: 89% and 75%, 83% and 74%, and 88% and 82%, respectively). Fourth, the predictive performance for adverse perinatal outcome of EFW < 10th percentile is very poor (26%, 9% and 5% for deliveries at ≤ 2, 2.1-4 and > 4 weeks after assessment, respectively) and this is improved by the proposed new approach (31%, 22% and 29%, respectively). CONCLUSIONS: This study presents an approach for stratifying pregnancies undergoing routine ultrasound examination at 35 + 0 to 36 + 6 weeks' gestation into four management groups based on findings of EFW and Doppler indices. This approach potentially has a higher predictive performance for a SGA neonate and adverse perinatal outcome than that of screening by EFW < 10th percentile. Copyright © 2019 ISUOG. Published by John Wiley & Sons Ltd.

THE PREVALENCE OF UNDERWEIGHT, OVERWEIGHT AND OBESITY IN A ROMANIAN POPULATION IN THE FIRST TRIMESTER OF PREGNANCY - CLINICAL IMPLICATIONS.

CONTEXT: Underweight, overweight and obesity are important global public health issues and risk factors for adverse perinatal outcomes. OBJECTIVE: To assess the distribution of the body mass index (BMI) in the Romanian obstetric population in the first trimester of pregnancy and its correlation with pregnancy outcomes. We also report the distribution of blood pressure (BP) parameters and their correlation with BMI. DESIGN: This retrospective study includes 9,064 women attending routine first trimester visit and ultrasound scan at 12.8(±0.6) gestational weeks. Characteristics, parity, method of conception, blood pressure (from 3,650 women), maternal weight and height, BMI and foetal ultrasound were recorded. Pregnancy outcomes were available for 1,607 deliveries. The Pearson correlation coefficient was assessed for each BMI group vs. blood pressure parameters, gestational age and birth weight. ANOVA analysis and post hoc tests were used to determine group differences. Linear regression was applied to estimate the contribution of BMI and gestational age to birth weight variance. RESULTS: In our population, 66.37% pregnant women had a normal BMI, 19.29% were overweight, and 7.56% were obese. There was a weak-to-medium positive correlation between BMI and blood pressure parameters, for all weight categories. The correlation between maternal BMI and birth weight was positive for normal and overweight. CONCLUSIONS: Our findings highlight the need for more effective health strategies targeting reduction of weight-related problems in women of childbearing age.

PLASMATIC LEVELS OF NEUROPEPTIDES, INCLUDING OXYTOCIN, IN CHILDREN WITH AUTISM SPECTRUM DISORDER, CORRELATE WITH THE DISORDER SEVERITY.

CONTEXT: Oxytocin has been investigated as a potential medication for psychiatric disorders. OBJECTIVE AND DESIGN: This study prospectively investigates correlations between oxytocin and other neuropeptides plasma levels in patients with autism spectrum disorders (ASD) according to severity and treatment, as compared to controls. SUBJECTS AND METHODS: Thirty-one children (6 neurotypical as control) participated in this study. The patients were classified into mildly and severely-affected, according to Autism Diagnostic Observation Schedule (ADOS) scores. Oxytocin, orexin A and B, α-MSH, ß-endorphins, neurotensin and substance P were investigated using a quantitative multiplex assay or a competitive-ELISA method. RESULTS: Plasma oxytocin levels differed between the groups (F (2, 24) =6.48, p=0.006, η2=0.35, observed power=86%): patients with the mild ASD had higher values of plasma oxytocin than those with the severe form (average difference=74.56±20.74pg/mL, p=0.004). CONCLUSIONS: These results show a negative correlation between plasma levels of oxytocin and the severity of ASD and support the involvement of oxytocinergic mechanisms in ASD.

Screening for pre-eclampsia at 35-37 weeks' gestation.

OBJECTIVE: To examine the performance of screening for pre-eclampsia (PE) at 35-37 weeks' gestation by maternal factors and combinations of mean arterial pressure (MAP), uterine artery pulsatility index (UtA-PI), serum placental growth factor (PlGF) and serum soluble fms-like tyrosine kinase-1 (sFlt-1). METHODS: This was a prospective observational study in women with singleton pregnancy attending for an ultrasound scan at 35 + 0 to 36 + 6 weeks as part of routine pregnancy care. Bayes' theorem was used to combine the prior distribution of gestational age at delivery with PE, obtained from maternal characteristics and medical history, with various combinations of biomarker multiples of the median (MoM) values to derive the patient-specific risks of delivery with PE. The performance of such screening was estimated. RESULTS: The study population of 13 350 pregnancies included 272 (2.0%) that subsequently developed PE. In pregnancies that developed PE, the MoM values of MAP, UtA-PI and sFlt-1 were increased and PlGF MoM was decreased. At a risk cut-off of 1 in 20, the proportion of the population stratified into high risk was about 10% of the total, and the proportion of cases of PE contained within this high-risk group was 28% with screening by maternal factors alone; the detection rate increased to 53% with the addition of MAP, 67% with the addition of MAP and PlGF and 70% with the addition of MAP, PlGF and sFlt-1. The performance of screening was not improved by the addition of UtA-PI. The performance of screening depended on the racial origin of the women; in screening by a combination of maternal factors, MAP, PlGF and sFlt-1 and use of the risk cut-off of 1 in 20, the detection rate and screen-positive rate were 66% and 9.5%, respectively, for Caucasian women and 88% and 18% for those of Afro-Caribbean racial origin. CONCLUSION: Screening by maternal factors and biomarkers at 35-37 weeks' gestation can identify a high proportion of pregnancies that develop late PE. The performance of screening depends on the racial origin of the women. Copyright © 2018 ISUOG. Published by John Wiley & Sons Ltd.

FETAL GOITRE IN MATERNAL GRAVES' DISEASE.

Fetal goitre is found in about 1 in 5,000 births, usually in association with maternal Graves' disease, due to transplacental passage of high levels of thyroid stimulating antibodies or of anti-thyroid drugs. A goitre can cause complications attributable to its size and to the associated thyroid dysfunction. Fetal ultrasound examination allows easy recognition of the goitre but is not reliable in distinguishing between fetal hypo- and hyperthyroidism. Assessment of the maternal condition and, in some cases, cordocentesis provide adequate diagnosis of the fetal thyroid function. First-line treatment consists of adjusting the dose of maternal anti-thyroid drugs. Delivery is aimed at term. In cases with large goitres, caesarean-section is indicated.

OXYTOCIN REDUCES SEIZURE BURDEN AND HIPPOCAMPAL INJURY IN A RAT MODEL OF PERINATAL ASPHYXIA.

CONTEXT: Foetal asphyxia, a frequent birth complication, detrimentally impacts the immature brain, resulting in neuronal damage, uncontrolled seizure activity and long-term neurological deficits. Oxytocin, a neurohormone mediating important materno-foetal interactions and parturition, has been previously suggested to modulate the immature brain's excitability, playing a neuroprotective role. Our aim was to investigate the effects of exogenous oxytocin administration on seizure burden and acute brain injury in a perinatal model of asphyxia in rats. ANIMALS AND METHODS: Asphyxia was modelled by exposing immature rats to a 90-minute episode of low oxygen (9% O2) and high CO2 (20% CO2). Control rats were kept in ambient room-air for the same time interval. In a third group of experiments, oxytocin (0.02 UI/g body weight) was nasally administered 30 minutes before the asphyxia episode. Seizure burden was assessed by the cumulative number of loss of righting reflex (LRR) over a two-hour postexposure period. Acute brain injury was assessed through hippocampal S-100 beta, a biomarker of cellular injury, 24-hours after exposure. RESULTS: Asphyxia increased both LRR and hippocampal S-100 beta protein compared to controls, and these effects were significantly reduced by oxytocin administration. CONCLUSION: Oxytocin treatment decreased both seizure burden and hippocampal injury, supporting a potential neuroprotective role for oxytocin in perinatal asphyxia.

Impaired placentation in women with chronic hypertension who develop pre-eclampsia.

OBJECTIVE: To compare the degree of impaired placentation in women with and those without chronic hypertension (CH) who develop pre-eclampsia (PE) in pregnancy. METHODS: Data were derived from prospective screening for adverse pregnancy outcomes in women with singleton pregnancy attending their first routine hospital visit at 11 + 0 to 13 + 6 weeks' gestation. This visit included recording of maternal characteristics and medical history and measurement of mean arterial pressure (MAP), uterine artery pulsatility index (UtA-PI), serum placental growth factor (PlGF) and serum pregnancy-associated plasma protein-A (PAPP-A). The measured biomarkers were converted to multiples of the median (MoM) after adjustment for pregnancy characteristics. MoM values in women with CH who developed PE (n = 283) were compared to those of women without CH who developed PE (n = 2236). RESULTS: In both groups with and without CH, measurements of MAP and UtA-PI were increased, whereas those of PlGF and PAPP-A were decreased and the deviation from normal in all biomarkers decreased with advancing gestational age at delivery with PE. There was no significant difference between women with and those without CH in the slope of the regression line of log10 MoM biomarker values against gestational age at delivery with PE for any of the biomarkers. However, there was a significant difference in the intercepts and coefficients of biomarkers in the two groups; compared to those without CH, MAP MoM, PlGF MoM and PAPP-A MoM were higher and UtA-PI MoM was lower in the CH group (all P < 0.01). CONCLUSION: In pregnancies that develop PE, the degree of impaired placentation, reflected in high UtA-PI and low PlGF and PAPP-A at 11-13 weeks' gestation, is less in women with CH than in those without CH. Copyright © 2017 ISUOG. Published by John Wiley & Sons Ltd.

Chronic hypertension and adverse pregnancy outcome: a cohort study.

OBJECTIVE: To examine the association between chronic hypertension (CH) and a wide range of adverse pregnancy outcomes after adjustment for confounding factors in obstetric history and maternal characteristics. METHODS: This was a prospective screening study for adverse pregnancy outcomes in women with singleton pregnancy attending their first routine hospital visit at 11 + 0 to 13 + 6 weeks' gestation. Data on maternal characteristics, medical and obstetric history and pregnancy outcome were collected. Regression analysis was performed to examine the association between CH and adverse pregnancy outcomes, including late miscarriage, stillbirth, pre-eclampsia (PE), gestational diabetes mellitus (GDM), spontaneous and iatrogenic preterm birth (PTB), small-for-gestational-age (SGA) neonate, large-for-gestational-age (LGA) neonate and elective and emergency Cesarean section (CS). RESULTS: The study population of 109 932 pregnancies included 1417 (1.3%) women with CH. After adjusting for potential confounding variables from maternal characteristics, medical and obstetric history, CH was associated with increased risk of stillbirth (odds ratio (OR), 2.38 (95% CI, 1.51-3.75)), PE (OR, 5.76 (95% CI, 4.93-6.73)), SGA (OR, 2.06 (95% CI, 1.79-2.39)), GDM (OR, 1.61 (95% CI, 1.27-2.05)), iatrogenic PTB < 37 weeks (OR, 3.73 (95% CI, 3.07-4.53)) and elective CS (OR, 1.79 (95% CI, 1.52-2.11)), decreased risk of LGA (OR, 0.65 (95% CI, 0.53-0.78)) and had no significant effect on late miscarriage, spontaneous PTB or emergency CS. CONCLUSION: CH should be combined with other maternal characteristics and medical and obstetric history when calculating an individualized adjusted risk for adverse pregnancy complications. CH increases the risk of stillbirth, PE, SGA, GDM, iatrogenic PTB and elective CS and reduces the risk for LGA. Copyright © 2017 ISUOG. Published by John Wiley & Sons Ltd.

Association of chronic hypertension with birth of small-for-gestational-age neonate.

OBJECTIVE: To examine the effect of chronic hypertension (CH), with and without superimposed pre-eclampsia (PE), on the incidence of a small-for-gestational-age (SGA) neonate and to explore the possible mechanism for such association. METHODS: Data for this study were derived from prospective screening for adverse pregnancy outcomes in women with singleton pregnancy attending their first routine hospital visit at 11-13 weeks' gestation, which included recording of maternal characteristics and medical history and measurement of mean arterial pressure (MAP). Birth-weight Z-score, adjusted for gestational age and maternal and pregnancy characteristics, and incidence of SGA were compared between those with and those without CH in the total population and in the subgroups of pregnancies with and without PE. Regression analysis was used to examine the relationship between MAP and birth-weight Z-score and incidence of SGA and PE in those with and those without CH. RESULTS: The study population constituted 74 226 pregnancies, including 1052 (1.4%) with CH and 73 174 without CH. PE developed in 233 (22.1%) cases of the group with CH and in 1662 (2.3%) of those without CH. In the group that developed PE, there was no significant difference for either median birth-weight Z-score or incidence of SGA between those with CH and those without CH. In the group without PE, the incidence of SGA was twice as high in those with CH than in those without. There was a significant association between log10 MAP multiples of the median and incidence of SGA and PE, which was more marked in those with CH than in those without. CONCLUSION: CH is associated with an increased risk of SGA and PE and this is related to MAP at 11-13 weeks' gestation. Copyright © 2017 ISUOG. Published by John Wiley & Sons Ltd.

Proposed clinical management of pregnancies after combined screening for pre-eclampsia at 35-37 weeks' gestation.

OBJECTIVE: To estimate the patient-specific risk of pre-eclampsia (PE) at 35-37 weeks' gestation by a combination of maternal characteristics and medical history with multiples of the median (MoM) values of mean arterial pressure (MAP), uterine artery pulsatility index (UtA-PI), serum placental growth factor (PlGF) and serum soluble fms-like tyrosine kinase-1 (sFlt-1), and stratify women into high-, intermediate- and low-risk management groups. METHODS: This was a prospective observational study in women attending a third-trimester ultrasound scan at 35-37 weeks as part of routine pregnancy care. Patient-specific risks of delivery with PE at < 4 weeks from assessment and PE at < 42 weeks' gestation were calculated using the competing-risks model to combine the prior risk from maternal characteristics and medical history with MoM values of MAP, UtA-PI, PlGF and sFlt-1. On the basis of these risks, the population was stratified into high-, intermediate- and low-risk groups. Different risk cut-offs were used to vary the proportion of the population stratified into each risk category and the performance of screening for delivery with PE at < 40 and ≥ 40 weeks' gestation was estimated. RESULTS: The study population of 3703 singleton pregnancies included 38 (1.0%) with PE < 40 weeks' gestation and 22 (0.6%) with PE ≥ 40 weeks. Using a risk cut-off of 1 in 50 for PE delivering at < 4 weeks after assessment to define the high-risk group and a risk cut-off of < 1 in 100 for PE delivering at < 42 weeks' gestation to define the low-risk group, the proportion of the population stratified into high, intermediate and low risk was 12.7%, 28.8% and 58.5%, respectively. The high-risk group contained 92% of pregnancies with PE at < 40 weeks' gestation and 73% of those with PE at ≥ 40 weeks. The intermediate-risk group contained a further 27% of women with PE at ≥ 40 weeks. In the low-risk group, none of the women developed PE at < 40 or ≥ 40 weeks' gestation. CONCLUSION: The study presents risk stratification of PE by the combined test at 35-37 weeks, aiming to identify a high-risk group in need of intensive monitoring from the time of the initial assessment and up to 40 weeks' gestation, an intermediate-risk group in need of reassessment at 40 weeks' gestation and a low-risk group that can be reassured that they are unlikely to develop PE. Copyright © 2017 ISUOG. Published by John Wiley & Sons Ltd.

GESTATIONAL DIABETES. OBSTETRICAL PERSPECTIVE.

This review discusses current international recommendations for GDM diagnosis and management and argues whether it would be worth considering first, universal screening for GDM in our country, second, updating of management guidelines and third, organized follow-up of women diagnosed with GDM and adoption of lifestyle interventions after delivery that could reduce the onset and prevalence of type 2 DM.

Gamma-ray bursts: huge explosion in the early Universe.

Long gamma-ray bursts (GRBs) are bright flashes of high-energy photons that can last for tens of minutes; they are generally associated with galaxies that have a high rate of star formation and probably arise from the collapsing cores of massive stars, which produce highly relativistic jets (collapsar model). Here we describe gamma- and X-ray observations of the most distant GRB ever observed (GRB 050904): its redshift (z) of 6.29 means that this explosion happened 12.8 billion years ago, corresponding to a time when the Universe was just 890 million years old, close to the reionization era. This means that not only did stars form in this short period of time after the Big Bang, but also that enough time had elapsed for them to evolve and collapse into black holes.

RAM-SCB simulations of electron transport and plasma wave scattering during the October 2012 "double-dip" storm.

Mechanisms for electron injection, trapping, and loss in the near-Earth space environment are investigated during the October 2012 "double-dip" storm using our ring current-atmosphere interactions model with self-consistent magnetic field (RAM-SCB). Pitch angle and energy scattering are included for the first time in RAM-SCB using L and magnetic local time (MLT)-dependent event-specific chorus wave models inferred from NOAA Polar-orbiting Operational Environmental Satellites (POES) and Van Allen Probes Electric and Magnetic Field Instrument Suite and Integrated Science observations. The dynamics of the source (approximately tens of keV) and seed (approximately hundreds of keV) populations of the radiation belts simulated with RAM-SCB is compared with Van Allen Probes Magnetic Electron Ion Spectrometer observations in the morning sector and with measurements from NOAA 15 satellite in the predawn and afternoon MLT sectors. We find that although the low-energy (E< 100 keV) electron fluxes are in good agreement with observations, increasing significantly by magnetospheric convection during both SYM-H dips while decreasing during the intermediate recovery phase, the injection of high-energy electrons is underestimated by this mechanism throughout the storm. Local acceleration by chorus waves intensifies the electron fluxes at E≥50 keV considerably, and RAM-SCB simulations overestimate the observed trapped fluxes by more than an order of magnitude; the precipitating fluxes simulated with RAM-SCB are weaker, and their temporal and spatial evolutions agree well with POES/Medium Energy Proton and Electron Detectors data.

Fronto-parietal connectivity changes following noxious stimulation during anesthesia.

RATIONALE: The aim of our study was to assess the changes in the fronto-parietal connectivity estimated by the cross approximate entropy (XAppEn) during noxious stimulation while under chloral hydrate anaesthesia, in rats. METHOD: A group of 11 Wistar rats chronically implanted with Ni-Cr electrodes, which were placed on the dura mater of the right hemisphere (over the olfactory cortex, the frontal and the parietal lobes), were used in the present study. Noxious stimuli of a mechanical and thermal nature were applied on the left hindpaw during chloral hydrate anesthesia. The anesthetic depth was estimated through median frequency computation, which in that instance was of 2-3 Hz. Fronto-parietal functional cortical connectivity was assessed by using XAppEn. RESULTS: After data processing and analysis we observed an increase of fronto-parietal functional connectivity during mechanical and thermal noxious stimulation. In addition, MEF increased both in frontal and parietal areas during the mechanical and thermal stimulation compared to baseline. CONCLUSION: Mechanical and thermal stimulation induces an increase in the fronto-parietal connectivity during chloral hydrate anesthesia in rats.

The bright optical flash and afterglow from the gamma-ray burst GRB 130427A.

The optical light generated simultaneously with x-rays and gamma rays during a gamma-ray burst (GRB) provides clues about the nature of the explosions that occur as massive stars collapse. We report on the bright optical flash and fading afterglow from powerful burst GRB 130427A. The optical and >100-megaelectron volt (MeV) gamma-ray flux show a close correlation during the first 7000 seconds, which is best explained by reverse shock emission cogenerated in the relativistic burst ejecta as it collides with surrounding material. At later times, optical observations show the emergence of emission generated by a forward shock traversing the circumburst environment. The link between optical afterglow and >100-MeV emission suggests that nearby early peaked afterglows will be the best candidates for studying gamma-ray emission at energies ranging from gigaelectron volts to teraelectron volts.

Decay phases of Swift X-ray afterglows and the forward-shock model.

The X-ray flux of the gamma-ray burst (GRB) afterglows monitored by the Swift satellite from January 2005 to July 2006 displays one to four phases of flux power-law decay. In chronological order, they are: the GRB tail, the 'hump', the standard decay and the post-jet-break decay. More than half of the GRB tails can be identified with the large-angle emission produced during the burst (but arriving later at observer). The remaining, slower GRB tails imply that the gamma-ray mechanism continues to radiate after the burst, as also suggested by the frequent occurrence of X-ray flares during the burst tail. The several GRB tails exhibiting a slow unbroken power-law decay until 100ks must be attributed to the forward shock. In fact, the decay of most GRB tails is also consistent with that of the forward-shock emission from a narrow jet. The X-ray light-curve hump may be due to an increase of the kinetic energy per solid angle of the forward-shock region visible to the observer, caused by either the transfer of energy from ejecta to the forward shock or the emergence of the emission from an outflow seen from a location outside the jet opening. The decay following the X-ray light-curve hump is consistent with the emission from an adiabatic blast wave but, contrary to expectations, the light-curve decay index and spectral slope during this phase are not correlated. The X-ray light curves of two dozens X-ray afterglows that followed for more than a week do not exhibit a jet break, in contrast with the behaviour of pre-Swift optical afterglows, which displayed jet breaks at 0.5-2 days. Nevertheless, the X-ray light curves of several Swift afterglows show a second steepening break at 0.4-3 days that is consistent with the break expected for a jet when its edge becomes visible to the observer.